This is a WIP map of a continent in my world:

Continent map

The small portion of land you can see in the topmost area is another continent, to the north of which the sea is almost always frozen and thus almost impossible to navigate.

The strait between the two continents is temperate, but on the other hand it's controlled by a single entity, which can impose trade tariffs as they please.

Since the entire eastern coast of this continent features an almost-impassable mountain range, traders coming from the east will probably be inclined to sail south and circumnavigate the continent, avoiding both the mountains and the tariffs.

The traders I'm interested in will be coming from another continent located to the east (not pictured). Their goal is reaching the continental area to the west of the mountain range (so, basically 95% of the continent) and a few more islands to the west. Picture this as some kind of Silk Road, where "fantasy China" is in the middle of this continent.

What could this southern sea be like, in order to discourage traders from trying to circumnavigate the continent, and instead force them to choose between paying the tariffs and trying to find a way through the mountains?

To be as clear as possible, my end goal is that the strait tariffs and the mountains will be the only viable solutions, not just preferable, and in the end due to political rivalry ("there's no way we will keep paying those tariffs!") the empire from which most of the merchants are coming from will decide to build a tunnel through the mountains, as in the other question of mine (linked), rather than having the possibility of just taking the southern route.

The more "original" (as in "woah I hadn't thought of that!", not necessarily something totally unique and never heard of) the idea - while preferably being simple to explain - , the better. So the best answers would either be ones with a lot of ideas, or a really cool and interesting one. In the end I guess the accepted answer will be the one containing the suggestion I end up using :)

Note that this is a medium-high fantasy medieval setting, so magic stuff isn't prohibited, as long as it makes sense (made-up megafauna = okay, "oh there's just a generic magic tornado always spinning around there" = meh)

The technology is comparable to 13th-century Europe.

What I have thought of so far:

  • dangerous megafauna (intriguing, but could pose more worldbuilding problems: why is it only in that area?)
  • ice (I would rather not; this area should have a temperate climate)
  • pirates (sounds a little dull maybe?)
  • storms

Note: Answers to this question surely help, but it's far more grounded and focused on realism, having the tag. In my case, fantasy explanations (as long as they're not far-fetched and are sensible - as in "can be explained so that they sound plausible in a fantasy world") are more than welcome.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Aug 11 '20 at 12:31
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    $\begingroup$ if this is an earth like planet the mountains may be impassible but the eastern side of them will have a narrow strip of land with some of the most fertile lands possible. while the western side will be almost nothing but desert. $\endgroup$ – John Oct 27 '20 at 1:50

28 Answers 28


Blue Reefs

The southern sea is big and open. The winds can go hard and the waves high, and there is no dry land nearby. This by itself would make the journey somewhat dangerous, but not especially so.

The problem is a certain type of coral reef that is common in the area. They form strong reefs with sharp edges, and have a deep blue colour - not exactly the same as the ocean around it, but close enough to be hard to spot. On a calm and sunny day, they aren't much of an issue. You can navigate around them, and they aren't close enough to the surface to reach you unless you sit very low in the water.

In less smooth conditions, it's another story. It doesn't have to be a storm, just a bit of wind and wave. The winds stirs up the surface making the reefs hard to see, and waves make valleys where ships go low enough to crash into them, making big tears in the hull.

Getting one or two tears in the hull is a solveble problem, especially if you're close to shore. Getting half a dozen new ones every few days, way out on the open sea, not as much. Even if you make it across without sinking, the repair costs would far outweigh the tariffs on the safer route. Worse still, the reefs break off and grow back in new places often, meaning maps go out of date in just a few years.

All this has naturally resulted in sailors considering the whole area cursed, so it's hard to get a decent crew together if you want to cross. Until you get iron hulls, best to just go the easy way.

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    $\begingroup$ This is at the same time original, interesting and simple. I also really like how the beautiful and harmless look of the brightly-colored corals (when the sea is calm) contrasts with how dangerous they are when the waters are agitated. $\endgroup$ – Hankrecords Aug 5 '20 at 14:42
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    $\begingroup$ to add to this answer, interesting plate tectonics could have left the area relatively shallow and with lots of sandbars that would make travelling across in anything but a flat barge very unlikely to succeed $\endgroup$ – Nullman Aug 6 '20 at 11:48
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    $\begingroup$ Or just shallow rocks would keep ships away too. A hazard doesn't need to be camouflaged for ships to want to avoid it. $\endgroup$ – Bohemian Aug 6 '20 at 23:53
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    $\begingroup$ This is an especially cool idea in my opinion not just because reefs are really fantastic and neat, but also because the implicit inclusion of magic into this concept means that it can be beautifully tweaked to be more or less extreme as the OP might require. Perhaps the reefs excrete a magical compound that dissolves things that touch the water above, for digestion? Carnivorous acid reefs. :) Tons of fun with this idea, since reefs are generally underrepresented in fantasy to begin with, and I've never seen any "magical reefs" before! $\endgroup$ – Onyz Aug 7 '20 at 15:18

Let's try a real world example

Say that the continent shown on the map is Africa. Traders coming from Shanghai in the east going to Amsterdam in the west have the option of going through the Suez canal and of course paying the transit fees, or going around Africa and avoiding those pesky fees. For some strange reason, they prefer paying to go through the Suez canal.

  1. Running a ship is not free of charge; the crew needs to be paid, supplies need to be bought, maintenance needs to be done.

  2. Ships are chartered for profit, not for fun. If going through the Suez canal means that a ship can do four trips against only two going around Africa, then paying the transit fees is better for both the shipping company and the merchants shipping the goods.

For an even more clear example, consider the advantages of transiting the Panama canal against sailing around South America. Besides making the trip a whopping lot shorter, nobody (see note) wants to go through the Drake Passage if they can avoid it.

Note: Nobody wants to go through the Drake Passage except adventurers who actually like being in danger on the high seas.

To summarize, the main reasons why the vast majority of shipping would avoid the southern route would be distance and wind patterns.

  1. Distance is the first obvious answer. You really don't want your ship to stay at sea more than necessary; merchandise in transit does not earn a profit; each day at sea increases the danger, and the expense.

  2. Wind patterns may be an equally powerful answer. Going around Africa west to east was very difficult until sailors learned that the correct route is to go all the way to Rio de Janeiro instead of trying to go around the coast. Trying to go around South America east to west was considered almost suicidal before the advent of steam engines. (In fact, sailing ships preferred to go around the world, from Europe to Rio de Janeiro to the Cape to Australia to Chile or Peru, instead of trying to go around Cape Horn east to west.)

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    $\begingroup$ It's in the question; "ice (I would rather not; this area should have a temperate climate)" But the roaring 40s itself presents a rather prosaic answer. At least for one direction of travel. $\endgroup$ – Jontia Aug 5 '20 at 11:16
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    $\begingroup$ @Hankrecords: The point of the answer is that the invisible hand of the market is a very very potent force. Ships can always go around, but they won't if there is a more economically efficient way. In the Antiquity, shipping companies preferred to transship goods over the Isthmus of Corinth (which was definitely not easy given the technological level) instead of going around Greece (which is much smaller than a continent). $\endgroup$ – AlexP Aug 5 '20 at 11:18
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    $\begingroup$ @Hankrecords: Distance is the obvious answer. You really don't want your ship to stay at sea more than necessary. Wind patterns are the second answer; going around Africa west to east was very difficult until sailors learned that the correct route is to go all the way to Rio de Janeiro instead of trying to go around the coast; and trying to go around South America east to west was considered almost suicidal before the advent of steam engines. $\endgroup$ – AlexP Aug 5 '20 at 11:24
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    $\begingroup$ Another consideration: ports. If there are all those big stonking mountains around the south shore, there may be few or even no towns of any real size. A ship probably could carry enough supplies to make that trip without resupplying, but that doesn't make it pleasant, efficient, or safe in the case of emergency. A trip past a shore dotted with ports is easier on the ship and crew, and less risky. $\endgroup$ – Cadence Aug 5 '20 at 17:56
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    $\begingroup$ @Brian if the port is operated by the state, rather than run as a private business, all you need is a stubborn politician intent on having a trade war and impose tariffs on the sailors. $\endgroup$ – Alexandre Aubrey Aug 5 '20 at 20:02

Teredo worms

The sea that lays to the south of your continent is cursed by having the ideal salinity and temperature for a particularly aggressive family of Teredo worms
While these creatures may also be present in other seas, and so known to your sailors, the species common in the South Sea make the long journey through these infested waters extremely damaging for the ships hull.

Teredo navalis is a very destructive pest of submerged timber. In the Baltic Sea, pine trees can become riddled with tunnels within 16 weeks of being in the water and oaks within 32 weeks, with whole trees 30 cm (12 in) in diameter being completely destroyed within a year. Ships' timbers are attacked, wrecks destroyed and sea defences damaged.

No treatment of timber to prevent attack by Teredo navalis has been completely successful. Experiments by the Dutch in the 19th century proved the inefficacy of linseed oil, metallic paint, powdered glass, carbonization (burning the outer layers of the wood), and any of the usual biocides such as chromated copper arsenate.

And this is the common Teredo.
The ones in your South Sea would be even more aggressive. A round trip in those waters would mean the destruction of the hull after just one travel.
The archipelago to the South East of your continent looks particularly suited for infestation of the coastal waters.
Outside of the South Sea they may not find the right conditions for reproduction (just like the real Teredo), thus limiting their spread in your world.
As they stick to the hull they also increase the drag in the water and slow the ship down. Historically ships had to visit the shipyards from time to time for scraping.

In 1878 it was discovered that creosote was an effective deterrent, though to work best it had to be applied to soft, resinous woods like pine; in order to work on harder woods such as oak, special care had to be taken to ensure the wood was completely permeated by the creosote.

Given the Middle Ages setting I would suppose your sailors do not know this method. Yet, in your story, you may have someone make the discovery making the South West Passage a possibility. Up to you.

If you would like to know more about these animals:
Smithsonian Magazine - How a Ship-Sinking Clam Conquered the Ocean

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    $\begingroup$ Very interesting! As these can grow to be a meter long (holy crap) I suppose 13th-century sailors would definitely notice their existence and not attribute the sinking of ships to cursed water/demons/whatever, which is also cool. $\endgroup$ – Hankrecords Aug 5 '20 at 16:17
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    $\begingroup$ Yes. They were well known since ancient time. The Smithsonian article mentions it. $\endgroup$ – Duncan Drake Aug 5 '20 at 16:21
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    $\begingroup$ Now make the worms eat flesh too! $\endgroup$ – Mad Physicist Aug 5 '20 at 19:23
  • $\begingroup$ Amazing. I thought Ensemble Studios made these up for the Age of Empires mission where you help Erik the Red get to Vinland, but apparently they are real. $\endgroup$ – Nzall Aug 6 '20 at 13:18
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    $\begingroup$ FWIW @Hankrecords I like these worms and the blue reefs best, and as noted by someone else, reefs rarely feature in fiction. So I want them combined :D Maybe the worms only live in the reefs? Or some waste product of the reefs causes the worms to grow much larger/hungrier/gives the worms the ability to eat treated wood/etc. Winds/currents could carry the waste product and/or grown worms out to sea, spreading the worm danger beyond shallow areas suitable for reefs. $\endgroup$ – user51614 Aug 8 '20 at 0:29

Trade winds

The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region (between 30°N and 30°S latitudes) (...)

Now check this out:

The Portuguese recognized the importance of the trade winds (then the Volta do mar, meaning in Portuguese "turn of the sea" but also "return from the sea") in navigation in both the north and south Atlantic ocean as early as the 15th century. From West Africa, the Portuguese had to sail away from continental Africa, that is, to west and northwest. They could then turn northeast, to the area around the Azores islands, and finally east to mainland Europe. They also learned that to reach South Africa, they needed to go far out in the ocean, head for Brazil, and around 30°S go east again. (This is because following the African coast southbound means sailing upwind in the Southern hemisphere.)

That is one hell of a detour - check on Google Maps - and it was still faster than following the coast.

In your fictional world, couple an easterly on the southern sea to an eastward ocean current and only motorized ships will be able to go west.

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    $\begingroup$ you don't need motors to sail upwind, just a lateen sail. it was a significant advancement in technology when it was invented, but old news by 13th century standards. sure, sailing with the wind using square sales is a lot easier than tacking into the wind, but both are preferable to the doldrums. $\endgroup$ – james turner Aug 7 '20 at 14:18
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    $\begingroup$ @jamesturner the portuguese did that turnaround in the 16th century. They had the lateen for centuries. It was still faster to use trade winds. It's a matter of economy. $\endgroup$ – The Square-Cube Law Aug 7 '20 at 14:28

Same situation as up north.

You are interested in a fiction with political rivalries. Your northern passage is controlled by a single entity. Duplicate this with the southern passage. The equatorial desert continent (not pictures!) approaches close to your fantasy China, and a power on the northern coast of this continent controls the shipping route.

It would be easier and more interesting to write about this than to explain to your readers about wind.

This also offers the possibility of merchants trying to play off the northern vs the southern power, trying to get them to compete to lower their rates. Unfortunately one group is less Suez Canal and more Tripoli pirates.

Side note - where are the inland waterways? Your inland empire will need water. If I were approaching from the west I would look for a big river and see how far inland it could take me.

  • $\begingroup$ Having a different faction controlling the southern passage is interesting indeed. I'm not sure which continent you mean when you mention the "equatorial desert continent" (to the south or the east?). My idea, though, was that there were no other lands to the south of this continent (none reachable by boat, at least... something like Antarctica might be there) so if the only obstacle is posed by pirates/corsairs of the southern faction, what's stopping the traders from just going even more south to avoid the dangerous coast? $\endgroup$ – Hankrecords Aug 5 '20 at 13:32
  • $\begingroup$ Regarding the inland waterways, I still need to figure those out (W. very much I.P. map) but the question is mainly on how the traders coming from the east would reach the western coast of the continent, so the various rivers and lakes aren't that relevant $\endgroup$ – Hankrecords Aug 5 '20 at 13:34
  • $\begingroup$ @Hankrecords - you state that north of the map is ice, and your continent is temperate. That means to be that the whole thing is north of the equator and so just south of your map would be the equator. If you put another continent down there you could constrict the water passage as you do at the top of the map. That assumes that you can be flexible as regards your geography. $\endgroup$ – Willk Aug 5 '20 at 15:03
  • $\begingroup$ I probably didn't explain myself clearly, but the land in the top of the map is just a tiny portion of another continent (think the Americas, where the main continent in my map is South America, there's a whole other continent to the north of that, and to the north of even that continent, there's ice). I picture the equator being a couple dozens degrees south of the strait where the tariffs are being imposed, while the southernmost portion of the continent pictured has roughly the temperatures of South Africa $\endgroup$ – Hankrecords Aug 5 '20 at 15:09
  • $\begingroup$ This was my thought--it's tariffs all the way down. Even if the southern passage isn't fully controlled, maybe there are only a few places that you can put in for water, or maybe you can try to slip by the local customs police but if they catch you they confiscate your whole cargo and your ship and throw you in jail for a few years. Maybe whatever you're trading in is illegal in that civilization...maybe there are sumptuary laws and you're dealing in purple clothing. Probably as soon as your folks build their passage through the mountains, they will start charging exorbitant rates to use it. $\endgroup$ – user3067860 Aug 5 '20 at 21:00

There are plenty of good mundane (as opposed to magical) answers, so I'll add a bit of magic reasoning to the mix

sailors couldn't really know what their longitude was in open water until the 18th century, so in your world they use magic for it. the southern part of the continent has something wrong with the magic field around those islands, the reasons could be one of many, like a natural hole in the "magisphere" or some mineral deposits that mess with magic or maybe even some sort of monster that affects the how magic works around it.

So you have a problem with magic in the area and now your magical longitude device does not function so what do you do? simple hug the coast so you know where you are, but you can't! have you seen those mountains and islands? the tectonics in the area are pretty crazy, it's very likely that there are shoals, sandbars, underwater mountains, etc... just under the surface, its just not safe sailing close to the shore which makes the whole endeavor extremely dangerous and definitely too dangerous for any commercial company to send trade ship through, the risk/reward is just not worth it


Gaseous Erruptions

Burried under the seabed are huge pockets of natural gas at high pressure. Regular small movements of nearby tectonic plates cause partial releases of gas that significantly reduce the density of water and hence the buoyancy of boats and people until the gas can disperse into the air.

The few sailors who have survived attempts to cross the Southern Seas before turning back report foul air and ships suddenly vanishing below the waves for no reason only to sometimes reappear a few minutes later with entire crews drowned as if the very souls had been sucked out of their bodies. God forbid you try to make passage during a thunderstorm as the sea burns around you as though Hell itself had taken to the seas against you.

In mediaeval times, the people have no way of knowing the cause of this so fear alone should keep all but the most desparate of fugitives out of the Southern Seas. Couple this with poor winds and currents to make headway slow i.e. the ship has to stay in the affected area even longer and you have a winner. Alternatively, just make the affected area so large that traversal without hitting a gas release becomes statistically impossible.

  • $\begingroup$ Do you have any source for places on Earth where this happens often, for reference? $\endgroup$ – Hankrecords Aug 6 '20 at 7:46
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    $\begingroup$ news.bbc.co.uk/2/hi/science/nature/8205864.stm and theguardian.com/environment/2020/jul/22/… show cases of gas leaks from this millenia. Not quite as bad as I am describing but a more significant gas leak such as one caused by tectonic motion or an accident while drilling for oil/gas can be enough to capsize or even sink an oil rig/oil tanker. I remember reading about this somewhere but cannot find it any more. That said, just the toxic fumes from gas release should be enough to kill most of the crew. $\endgroup$ – Nicholas Adams Aug 6 '20 at 8:12
  • $\begingroup$ thank you, that helps! $\endgroup$ – Hankrecords Aug 6 '20 at 8:15
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    $\begingroup$ +1. This has been offered as an explanation by some people for some of the Bermuda Triangle's mysterious disappearances. $\endgroup$ – Wayne Aug 8 '20 at 15:56

Something Cool

The island looks like a tooth right? In fact it IS a tooth! It is the tooth of a very large and very slow-moving creature. Your world is a world of microscopic people living inside the mouth of an animal. They sail their ships through the saliva seas!

The South sea is where the tooth joins the gumline. Down there are giant predatory plaque bacteria. Those guys like to eat ships and people booga booga booga! No one wants to go down there.

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    $\begingroup$ Or, that's what your people believe... $\endgroup$ – Shawn V. Wilson Aug 6 '20 at 22:30

The first thing I thought of when I read this was the Sargasso Sea. Have such a high amount of seaweed in the seawater there that it's nigh on impossible to sail a ship through it. You might need to make it slightly warmer than temperate, but it would seem to fit with the geography for it to be warmer.


Some possibilities include

  1. Prevailing winds make it too difficult.
  2. The region is a doldrum without winds.
  3. The region is dead, magically, and the ship requires at least one spell to function properly, or at all.

Nobody knows.

No ship that's ever attempted the Southern Passage has ever survived. Since no ship ever survives, nobody knows what is killing then.

(Obviously, this isn't a really answer, but it could be a fun way to add some mystery tip whatever answer you decide to use. Also, it allows you to use a reason that has an obvious work-around, because sailors can't take advantage of it if they don't understand the danger. Of course, readers may expect you to pull on the thread and explain the mystery effectually)

Virulent diseases

The sea birds off the Southern coast are carriers for a lethal disease which can sicken and kill an entire crew.


The empire that controls the strait cursed the southern passage to give them a monopoly on trade.

Alternately, the people who dwell (or dwelled) in the southern mountains really don't like visitors.


Any ship that takes the passage is led to its doom. Probably either a curse or a cannibalistic society, as natural sirens might be tricky to justify.

Takes too long.

Sailors can only live for a certain period without eating fresh surtic fruit or they come down with magical scurvy. That's long enough to take the strait, but not long enough to take the southern passage, and surtic fruit doesn't grow down there.


There's a giant rift in the ocean in that part of the world

It's a sort of edge-of-the-world kind of thing. There's a 2-3 mile wide gap along that part of your world where there is no ocean and the water that forms the walls of the chasm is magically suspended. Basically, it's a giant canyon made of water instead of rock.

It's not a matter of danger. The ocean just doesn't exist there, so there's no possible way to sail across.

An example of this can be found in Ni No Kuni 2: Ni No Kuni's Ocean Rift


Religious Taboos

Never underestimate the power of superstition and/or religion to keep people from doing something. In your world, the dominant religion has placed taboos on the Southern passage and any sailor who dares traverse it is faced with excommunication (or a similar punishment).

Why would they? I can think of a couple of reasons, and there are probably more:

  1. Rumors of mythical beasts. Legend has it that a sea serpent, Kraken, or similar monster lurks in the Southern passage. This alone would be a powerful deterrent, but your religion considers it the devil incarnate out to steal the souls of fools who pass that way (or something similar).
  2. Corruption within the religion. The religious leaders have cut a deal with the being controlling the northern passage, who has bribed them handsomely for their services.

Now, you specified that you want it to be impossible, not just unlikely, that they would take the Southern passage, and even if it seems possible, it really isn't. Even with a skeptical captain who is willing to risk the voyage, no crew would sign on to such a ship, and if tricked into it would almost certainly mutiny.

I hope this helps you.

  • $\begingroup$ This would be an interesting take, but I'm afraid it's just not enough of a reason for traders to completely avoid the area. Not all people on the eastern continent follow the same religion, and even if they did, history is full of broken taboos: Greeks thought the strait of Gibraltar (the pillars of Hercules) were not to be traversed as it would be an insult to the gods, but many did so anyway. $\endgroup$ – Hankrecords Aug 7 '20 at 9:11
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    $\begingroup$ Worth a shot. 👍 My reasoning with them not breaking the taboos is that some who wouldn't fear the dangers or anger of the Gods would still hold back because of excommunication, not wanting to ruin their afterlife. But yeah, it might not be enough. $\endgroup$ – Benjamin Hollon Aug 7 '20 at 9:27
  • $\begingroup$ What if it was not just superstition but acts of god(s) that make the passage impossible. (Gods in some worlds do intervene when the people do not follow the rules.) $\endgroup$ – Willeke Aug 7 '20 at 15:16

See those islands off the south-east of the continent? They extend a long way south. Way further than can be seen on this map.
They are a volcanic archipelago with many active volcano's both above and below water. Charts can not be relied upon. The reefs and the shapes of the islands change all the time because of the eruptions. Ash-fall, explosive eruptions and earthquakes (with tsunamis) happen often.

Passing through the archipelago is extremely dangerous. No matter how much money you pay the crew, as soon as you try to go through those islands you'll have a mutiny on your hands.

So you have no other option to go north or make a very long detour south with makes the southern route almost 3 times as long. (Especially if the point of origin in the east is at roughly the same latitude as the northern tip of the archipelago.) Prevailing winds in the southern seas are westerly so they don't help either.

Carrying enough fresh water on-board for the longer southern route is a problem and cuts into your cargo capacity.
Cargo "on the move" earns no money. The shorter the transit time the better.

Shipping costs for the southern route are just not economical.



That's as far as I'm going on that subject.

There are a wide ranges of things that will keep a ship out of an area. One of the big ones is lack of access to accurate navigation. You have to keep well clear of any known reefs and rocks because you don't really know where you are until it's possibly too late.

Your map shows a line of (rocky) islands off the South East coast for example: the winds, tides and currents around those will be a nightmare for a wooden sailing vessel. You'll have patchy wind being focused between the islands, you'll have the same in currents, potential whirlpools if you catch the tide wrong (see sea crossing from Scotland to Orkney for example) unknown draft, sharp rocks, better just to stear well clear of the whole area.

The Southern Ocean is another real world one. The northern oceans are surrounded by continents, storms and waves can only get so big. The Southern Ocean goes uninterrupted all the way round giving the waves space to keep rolling and keep building. The places where it's interrupted by land, such as Cape Horn and the Cape of Good Hope are famed for the sea conditions one encounters. Remember the Cape of Good Hope used to have a simpler name, the Cape of Storms.

Ocean currents are an interesting one, consider the gap at the northern end of your continent. There's going to be a strong current passing through there, probably East to West with the tide. At certain points of the tide it'll be faster, sometimes slower, but as a general rule it'll flow East to West. That means if your ships are a bit slow, like the Spanish Armarda, they'll not be able to sail East through that gap. I mention the Spanish Armarda specifically, because after their battle with Drake in the Channel, to get home they had to sail North round Scotland and Ireland to be able to get home. The current in the Channel is primarily northbound and the ships were too slow to sail directly South.

Now some of these aren't areas of sea to avoid specifically, they're constrictions on which direction you can pass through certain regions, but they should be touched on, along with the reminder that there's a right way and wrong way to sail round the world.


We are ovethinking this. How about. . . .

The South Sea is Dangerous.

The North Sea is calm and sheltered. The South sea has frequent storms and wrecks. Hence traders refuse to go south.

But Daron, I wanted the tunnel to be the only viable solutions, not just preferable!

For one person making one trip the South sea is an option. However since most traders refuse to use that passage, making a tunnel is the only option for the kingdom.

Consider how merchants operate. They own a ship and sail back and forth and hopefully make the trip dozens of times before they retire. Since there are so many trips even a 10% death rate is enormous. Heck even a 10% lose-all-your-goods rate is enormous. It means you need to make more than 10% profit on each trip!

If I was on the run from the baddies I might risk that 10% rate since I'm only going there once . But if I was a trader I would refuse to use that route.

Bonus: If you want something more exotic you can have that too. Since people rarely go South and dead men tell no tales, there very well MIGHT be a Kraken down there.

  • $\begingroup$ This is surely a sensible reason for why they would prefer the tunnel, but my question is specifically about what makes the southern sea so dangerous :) $\endgroup$ – Hankrecords Aug 5 '20 at 16:33
  • $\begingroup$ @Hankrecords It is dangerous because of storms. $\endgroup$ – Daron Aug 5 '20 at 16:35
  • $\begingroup$ You may be severely underestimating the profit margins involved. In real life, the Indian Ocean trade in the ancient and medieval periods led to a huge eastward gold drain due to the prices involved. This answer on history stack excahnge seems to suggest that profit margins might easily be several times the costs. $\endgroup$ – Devashsih Kaushik Aug 6 '20 at 13:51
  • $\begingroup$ @DevashsihKaushik Yeah I don't know if 10% is a high profit margin in the age of sail. $\endgroup$ – Daron Aug 6 '20 at 13:59

Very high Shipworm concentration

With your southern islands producing a lot of natural flotsam of trees (maybe from constant storms), the incidence rate of shipworms is astronomical in the southern waters. Normally taking about a year to cause sufficient damage, the species present in your waters reproduce more quickly and are of an abnormally large size. Ships will be infected and made non-seaworthy in the a shorter period of time than it takes to make the journey.

Combine the weakened hulls with higher storm rates (also the reason why so much timber is floating around) and you have an impassable sea from plausible factors.

Some fun plot ideas:

  • Certain types of wood (or metal plating) could be used by others to allow passage.
  • West to East passage could be allowable while restricting East to West travel if you make the concentration of shipworms only in the eastern portion.
  • Some people could risk it and have their ships eaten out from under them halfway through the voyage to report upon this infestation.
  • Ocean salinity is an important factor of where shipworms can live, this could also be used to designate safe/unsafe regions.

Certain geological activity can make that part of the sea too dangerous to use.

The continental shelf to the southeast of your continent could have large natural gas deposits fairly close to the surface. Ridges throughout the area could have unpredictable eruptions of methane gases. These gases are lighter than water and create a region of the sea that suddenly becomes highly aerated and is far less buoyant than regular seawater. Any ships caught in such a region would sink quickly, and for no apparent reason.

It doesn't take many ships suddenly disappearing in calm weather before sailors will go out of their way to avoid the area. With no way to predict when or where the sea will bubble, few will risk that route. Losing a single ship with a full cargo hold will cost you more money than several years' worth of tariffs from the northern route.

This is actually one of the proposed explanations for ship disappearances in the Bermuda Triangle.


One factor no has mentioned, yet is obviously simple: lack of harbors or ports.

As we all know, until the industrial age ships could not stay at sea safely for extended times. (Even in the industrial age, they needed land bases where they could restock coal or coal to drive their engines.) At regular distances, a ship would need to land somewhere to replenish its supply of food & water, maybe replace sailors who were sick, injured, or had died. They can only do this where there are established port towns or cities with a hinterland to provide these goods. (A bay or open roadstead alone will not meet this need; even one with a good hunting ground will eventually be denuded of all of its game &/or vegetation due to overuse, & become a barren stretch.)

This ties into the plausibility of your "almost impassible mountains": given enough incentive, any mountain range is passable. There were trade routes across mountainous Ethiopia with its deep canyons, & over the Himalayas the tallest mountains on Earth, all because merchants could make a good living freighting goods across these obstacles. However, if there were no available ports on the seaward side of these mountains -- or all of the known ones were controlled by that empire to the North -- this would serve as a sufficient disincentive to traders crossing these mountains. (Or were they to cross them, the tariffs at the ports controlled by the hostile power would eat up an unacceptable share of any profits. Or the garrisons would simply confiscate the trade goods & turn the traders away.)

With this comes a possible plot device. One of the good guys -- traders from the Easterners or the Westerners -- discover an overlooked port on the coast which can be connected by building a relatively short tunnel thru the mountains. By this, instead of one several miles/kilometers long, say long enough to be completed in a year or so. This would be a short enough time that by the time the hostile power to the North learned of this, mobilized a fleet & made an attempt to capture this port, the tunnel would be completed & good guys could respond much more quickly with enough troops to defend it. Time would be of the essence here: if the tunnel (or maybe a bridge, or maybe a combination of these) is not completed in time, the adversaries will get there first, & not only fortify it but march inland to slaughter the engineers (or navvies or sappers) constructing the bridges/tunnel.

This does take the scenario out of the realm of fantasy, but it would still be a fascinating adventure story.


Fairly straightforward:

North of the continent the waters are shallow because the two continental landmasses have quite a small gap, technically they're the same continental plate. The shallow and comparatively sheltered strait between the two continents has quite mild weather suitable for sailing.

South of the continent, the sea currents are powerful and mostly flow east. The south is hard to travel at the best of times, and the combination of the mountainous coast of the continent, strong winds and ocean currents and open ocean produce frequent powerful storms that make traversing the south even harder.

North is the only sensible route.


Here there be merfolk

Some sort of sapient aquatic or amphibious people occupy the southern seas and don't like merchant vessels.

After decades of surface people using the southern ocean as a dumping ground for refuse, mutineers, and colonial agricultural runoff, the sea peoples have vowed to stop all sea traffic into the region. The waters between the southern islands and the mountainous coast were naturally shallow, with submarine volcanoes and isolated seamounts, and are now built up with surface refuse and quarried rock. Sandbars are dismantled, moved kilometers; wooden wrecks are tethered to the seafloor and allowed to buoy just a meter under the water surface, ready to gut any night-going ship. It might be possible to sail through this region with an up-to-date map of all the shifting shoals, but add in the sea peoples' own formidable weaponry, their cultivation of predatory megafauna, and the terror of wild speculation, and these days few pilots dare to even anchor on the southern islands, let alone venture past them.


Two things

  1. Already mentioned were trade winds; if they were pretty consistent/strong westerlies that blew for most of the year then travel to the west could be prohibitively expensive/time consuming for sail powered vessels given they have to travel down the coast of the continent first. So coming back that way might be fine but getting to the West coast via this route in the first place - not so much. So the route is just not particularly attractive cost wise. (Especially if the nation that imposes tariffs for using the other route has carefully calculated how much they can get away with charging before the other route starts to look attractive.

  2. Endemic piracy - you've already noted there are few if any towns along the East coast with access to routes over the mountains. This doesn't mean those towns/cities might not be worth while trading with on their own BTW but that is not your objective.

However it may be that some of the islands to the south have few if any natural resources and are ruled by petty feuding kingdoms or tribal groups that have a long history of piracy and raiding to supplement their otherwise meager incomes. If there is no strong regional power on the mainland or elsewhere nearby with a strong incentive to suppress their activities the southern route could simply be too chronically dangerous for anything but largest most well armed merchant fleets traveling in convoy (also very expensive).

More likely both combined would be the explanation.


The Southern Seas are the ruins of an Ancient Continent destroyed in Hoary Antiquity

This is actually a concept with a precedent in, of all things, A Song of Ice and Fire. The ancient peninsula, was literally shattered, and the sea formed by its smoking ruins is the site of permanent volcanic activity and thought to be infested by demons. Indeed, as per The World of Ice and Fire (quoting from the wiki):

In 55AC, Princess Aerea Targaryen claimed Balerion as her mount, and disappeared for more than a year, before reappearing in King's Landing in 56AC. Grand Maester Benifer and Barth found her to be infested with "horrors" which burned her from the inside out and killed her, erupting from her body as "worms with faces", "snakes with hands" and other monstrosities. Balerion was also found to bear wounds and scars seemingly received during their absence. These observations led Barth to the conclusion that Balerion, uncontrollable by an inexperienced rider such as Aerea, flew her to his original home of Valyria, where he was born before the Doom and the flight of the Targaryens; these "horrors" that infested Aerea, he supposed to be remnants of Valyrian experiments with blood magic. After these events it was declared forbidden for any ship suspected to have visited Valyria to dock in Westeros, or for any inhabitant of Westeros to travel to Valyria.

Not only is this a reasonably established trope in fantasy, such stories are present in real life too, such as the fabled lands of Atlantis and Lemuria, and the myths form a familiar part of pop-culture; being referenced in literature, occultism, politics, and even music.

There are several ways in which this could work for your purposes:

  1. A large area south of the continent could be a shallow wasting sea with frequent earthquakes sending tsunamis into the entire southern sea and frequent fiery eruptions making the crossing even more hazardous. You can optionally add eldritch monstrosities to increase the danger. To explain why the creatures are limited to the area, use either the presence of special magic required by the monsters (or keeping them at bay), or their need for extreme heat provided by the incessant volcanic activity.

  2. Have the continent be destroyed in an apocalyptic explosion or temporal cataclysm, which has rendered that part of the sea much deeper that the surroundings. You could have warm water from the surface sinking into the cold depths throughout the tropical ocean, leading to strong currents into the sea from all sides, trapping anyone who ventures into the region. Alternatively, strong volcanism at the seafloor could lead to rising waters and strong outward currents denying entry to the region. Either way, the limited technology would make the ocean innavigable.

  3. Let the people of your fantasy-India be survivors from the destruction of the southern land (or alternatively their ancient enemies). They could have a strong taboo against mixing with people who have passed through the sea. Maybe the authorities have even banned ships coming from that route from docking anywhere in the country. This would easily make the ocean anathema to all traders.

  4. Another option might be to use the warmth from the volcanism (or magical forces) to cause unpredictable super-cyclones that ravage the sea and crash into the mountains at the south of the continent. You could have synchronized activity where there is volcanism near the continent much of time, but then suddenly it shifts to deeper waters in the South leading to cataclysmic storms being triggered out of the blue. The double whammy would increase the aversion even more.

If you want to keep things science based, an asteroid impact (think the [Chicxulub Impact]), or ongoing seabed volcanism a la' the Siberian and Deccan lava fields. The magma plume that caused the Siberian flow went on to cause activity in the Arctic Ocean, and all these events are thought to have led to mass extinctions. It should not be difficult to tune the timing and intensity to suit your needs.



You ask why would megafauna be found more often in the southern ocean; real ecosystems provide some options

Ocean Upwelling

Ocean upwelling is a circulation which brings nutrient-rich water to the surface resulting in a larger populations of photoplankton which in turn support the rest of the food chain.

Approximately 25% of the total global marine fish catches come from five upwellings that occupy only 5% of the total ocean area. (Wikipedia)

Upwelling often occurs in coastal regions. One example, is the California Current. In your geography, the southern coast may have more upwelling than the northern coast. The fish support a thriving population of sea monsters which are very territorial and destroy transiting boats. For this option, I would suggest giant vertebrates such as whales, or aquatic dragons, rather than giant cephalopods. For cephalopods, see the next section.

Deep Sea / Polar Gigantism

For unknown reasons, deep sea and polar marine invertebrates tend to be larger than their shallow water relatives. A well-know example is giant squid. Hypotheses for possible causes include lower water temperatures increasing cell size, less population pressure, and scarce food resources (Wikipedia).

Regardless of the actual reason, you only need to know that size is correlated with water temperature and depth. The southern ocean is deeper and colder than the northern ocean and therefore has large and dangerous invertebrates, such as the Kraken.

Why are these deep-sea creatures harassing surface-going ships? Some cephalopod species gather in spawning events. Perhaps these Kraken gather near the surface for reproduction and mistake the ships for competitive males.

Wildlife refuge

Another reason for megafauna population differences across ecosystems is human-caused population pressure. In the north, there is clearly a large human population, hence the tariffs. The southern ocean is a marine protected area for endangered megafauna species. Therefore, not only are dangerous megafauna present, but it is illegal for commercial activities to take place there, especially during breeding season. The traders are not only tax-dodgers but environmental criminals.


You said pirates are mundane, but what if there is a single area in the southern part of the continent that's suitable for a port. And it has a port. An exotic port with multiple ships and crew and taverns and supplies... and it's all fake. Everyone you meet is a resident of the town and they put on an elaborate charade for crews who visit. (They can see ships far off from a perch in the mountains, and prepare for them.)

Ships are sailing against incredible currents and winds and need to stop somewhere. And when they stop at this welcoming port, the entire crew is zombified (drugs in their drinks at the tavern) and put to slave labor in mines etc.

The ships are modified to be hard to recognize and then play their part in making it look like a crowded port. They're also used for actual trade with the western side of the continent. A ship can make it from the port to there by hugging the coast for about the maximum viable sea journey.

So this southern port is fabulously wealthy: they get free ships and cargo from unsuspecting merchant crews, they get free slave labor, and when they show up on the west coast and run into Eastern traders who took the northern route, they can let it leak accidentally that they took a southern route -- recruiting more victims. Oh, and some of them do actual trading, just with other people's ships and cargo.

With all of those ships and a hidden lock on the southern route, they could assemble a bunch of cannons in a fortress guarding their town. ("Pirates around here. I hear some ships disappear after they leave us. Gotta be careful.")

So it is pirates, but a much creepier version. (Of course, the Eastern Kingdom traders would never know this back story. They would only know the "pirates" story, or perhaps they'd be told, "Ah yes, I remember that trader. He partnered with Josiah's son and headed to the far west islands, where you can make an unbelievable fortune in a few years! That's how I got my place here: took some risks in the far West for a while and finally retired here.")

So in some sense, it's piracy on the northern route and the southern route, just different kinds: on the north it's civilized and open and doesn't really seem like piracy at all, on the south its hidden and sinister and also doesn't appear to be piracy at all.

(If you want, make them vampires who really only need the blood of the crews that stop, and not food or other supplies. May or may not be creepier.)


I'm going to post a tangent answer.

One of the most compelling theories for Werewolves and 'Beasts of the Full Moon' is that nocturnal predators (like wolves and jungle cats) are always at their most desperate/hungry during the times around a full moon - because their prey have an easier time seeing their approach. And that predators like this, which would ordinarily avoid trying to take down dangerous human prey, are desperate enough to eat that they attack humans.

Why am I mentioning this?

There are still people that say things like, "Uh oh, full moon tonight; the crazies are going to come out" - there's still a concept that the full moon means danger - and this is hundreds (or even thousands) of years after 'wolf attack' was a typical existential threat to the average person.

So simply put... there doesn't have to be anything at all down to the south. Maybe there was something in the past, something terrible or pervasive or comprehensive... but at this point, it's halfway into the realm of myth/superstition.

Sure, there might be some brave souls that go the southern route to avoid the tariffs, but "most sensible people realize there's no point in tempting calamity just to avoid paying a few coins."



For example, look at the spice trade in the Strait of Malacca. Boats coming from India, laden with spices for sale were a prime target for pirates... and traders would steer well clear of these areas.



C.S. Lewis's Voyage of the Dawn Treader gives several fantastical impediments to sea voyages that you could use as inspiration and modify:

Magical sleep curse

One of the islands is cursed in a way that eventually causes an endless sleep. This could in your case apply to the entire sea in such a way that no one can cross the sea because no one can stay awake long enough to navigate through it. Similar ideas would include magical confusion, maybe even rage inducing confusion causing crew members to always mutiny or just kill each other, or go the other way and cause extreme placidity such that they just have no desire to do anything, even survive. Or both at the same time (Miranda...).

Water that transforms everything to Gold

This is actually by itself not too bad, if you can stay in the ship, not touch the water, and the alchemy of the hull doesn't sink the ship (A ship that could survive this wouldn't be too difficult to build). However, if even the sea mist turns things to gold, then eventually the crew would start becoming effected. On the ocean it's really hard to keep things dry, especially in storms.

Gold though would be almost as much of a draw as a deterrent though, but you could do various other types of alchemical effects of things that entered the area, including things that would affect mostly ships, but not people (makes ships brittle, less tough, or very soft, turns wood to mercury or gallium, which soon melts). A real world similarity is just radioactivity. A highly radioactive sea would be for all intents and purposes impassible without extreme engineering.

A cloud of nightmares

Areas where your dreams or stray thoughts becoming manifest as you think them. Could be passible, but the extreme mental discipline required to not think of deadly pink elephants would be hard to find in a ship's crew. But maybe some crews do exist, and they could even use this to their advantage, and use it to prevent unwanted competition. That could be your "pirates", but they would be very different from the rowdy scoundrels most people imagine pirates as... These "dream clouds" could also be a kind of local "weather phenomena" in behavior. Making it difficult to avoid, and too dangerous for someone not trained to even attempt.

  • $\begingroup$ Anybody care to explain why the down vote? I'd like to fix it if I can. $\endgroup$ – Garrett Motzner Aug 10 '20 at 17:07

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