What scientifically would make it possible that certain large bodies of water be impassable?

Question

What scientifically would make it possible that certain large bodies of water be impassable while others that seem very similar are able to be navigated?

This is not what I am attempting to do, but as an example say in some sort of alternate history no one discovered the Americas due simply to the fact that their vessels could not cross the Pacific or Atlantic Oceans. There were ships that could cross the Indian Ocean perhaps, but not the two that surround the 2 western continents. This would obviously be made possible by further future development, but say until the 1700s it was not.

Is this at all possible? I'm not looking for an answer that they didn't know the continents were there, but rather there is no way they could have known because navigation across those two bodies of water was impossible once a certain distance from Europe, Africa, or Asia was reached.

Answer 1

The winds

Very unfortunate wind patterns are going to make it very difficult, if not impossible to cross an ocean with sail ships, requiring a civilization to invent steam power or sufficiently advanced sailing techniques (yes, it is technically possible to sail against the wind) to cross the ocean.

It is entirely possible for wind patterns to be permanent, for example the westerlies, going only in one direction. That can make an ocean impassable, though this mechanism is direction sensitive.

Permanent wind patterns can also isolate an entire continent, through a mechanism like the polar easterlies, caused by air sinking over the continent and subsequently move in any direction.

Visualization of some of the permanent wind patterns on Earth:

Answer 2

Shipworms

They're real: https://en.wikipedia.org/wiki/Teredo_navalis

and they'll eat through the hull of your wooden ship in a matter of weeks. Make them a little bigger, a little hungrier, and long crossings will be impossible.

Even until quite recently a lot of ships were lost at sea, never seen or heard from again. The Titanic is unusual, not because she was lost, but because we know where and when and because there were actually survivors. Normally, even at that point, a ship lost at sea would set sail and just never turn up again.

Your ships will go to sea and the shipworms will start eating. The hull will be gone before they see land again and long crossings become impossible, but as requested, later technology makes the crossings possible (though later than suggested):

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...[snip]...The only permanent solution to attack by Teredo navalis, however, is to replace wood in submerged constructions with some other material.

or dragons, you can always have dragons.

Answer 3

Here's an interesting one: Bubbles would do it. Bubbles rising through water make the water less dense. Any ship will lose buoyancy and sink into the frothy water. A curtain of bubbles would be an impassible barrier.

The problem is that it's hard to come up with a scientifically credible source of enough bubbling that could last for hundreds of years. If you're willing to hand-wave the details and plausibility, you could just say there's an underwater volcanic ridge spewing gas bubbles. Melting ice on the sea floor could also release bubbles, although that explanation is probably an even bigger stretch.

It may be more plausible if you have occasional random patches of bubbles erupt. That would make sailing the area risky. People would simply stop sailing the danger zone after the first few ships were lost.

Answer 4

Europeans brought measles and smallpox to the Americas and an estimated 90% of the native population died. Now suppose that there had been a disease to which Europeans had no immunity....

In your world there is an island continent which it is almost impossible not to arrive at. The natives are friendly and compassionate. The water, however, is deadly. It contains a parasite which is like Cholera but even more deadly, to which the natives have acquired resistance. (In our world if you have blood group AB you are extremely resistant to cholera).

Ships land, are welcomed, replenish their water. Then everyone becomes ill. Most die. There are too few survivors to crew a ship. Add to this, for the conscript sailors there is little desire to try to return. The natives are very friendly, and the women are beautiful. They have cheated death and found a paradise!

So nobody returns and soon it is well known back home that this particular ocean is impassable. Attempts to prove otherwise will be rare and if made, the same fate will descend on the next expedition.

Answer 5

There are a few problems with early trans-atlantic voyages. For instance:

• Preserving enough food is troublesome. Commercial refrigeration wasn't technically available until 1755. Only traditional techniques could be used to preserve food, and it's hard to say how much food to pack when traveling into the unknown. Famously, Christopher Columbus' first voyage had concerns about their food and water supplies lasting for a return voyage. The obvious solution is to simply stock more food and water, so this is a weaker solution.
• Columbus (and those who came after him) benefited from the sea currents in the Atlantic. This is why he was deposited in the Caribbean instead of what is now the US eastern seaboard; the water literally pulled his ship there. If the sea currents were incorrect, then it would prevent many ships from effectively traversing the sea in question. If the currents were not helping, the voyage by sail could be impossible.

Some other factors to consider in general navigation:

• Hurricanes / Monsoons could easily stop and destroy ships from crossing a particular bit of ocean. Of course, the sea in question would have to have some method of developing regular, intense storms.
• Heavy Tectonic Activity could make strong waves, and a varying sea floor depth could cause these waves to become tsunamis. Obviously, the sea floor rising would have to form atoll-like structures or otherwise possess very shallow seas next to very deep seas for these large waves to form dangerous tsunamis. (A tsunami at sea is actually a very long, but very gentle, wave.)
• Sea Ice can cause a sea to become very dangerous, especially if the sea ice shifts and changes very frequently. The frequent change would be required to prevent people from simply walking along the surface, certainly much faster than the arctic does.
• Fata Morgana can lead sailors deeper into the ocean, thinking that they've finally seen the land their looking for. These can come from ice bergs or other oceanic bodies.
• Severe temperatures can prevent people from crossing a sea. It could be simply too cold or hot, especially if the people cannot gain access to proper insulation.
• Predators could prevent ships from crossing. If Dunkleosteus or other large sea predator was still around, it could try to take a bite out of a columbian-era ship. For scale, consider the following:

and the fact that the Niña was ~15.24 m (~50 ft), somewhere in the low end of Megalodon range. A megalodon-like creature could try to defend its territory, and possibly be like a dunkleosteus and simply be used to chewing on hard things.

Answer 6

Limit your sailor's navigation abilities

Although it would not make navigation fully impossible, you could consider giving your world very limited visible stars, and weak (or no) magnetic fields. The primary tools for determining absolute position in the 17th century were the astrolabe and the magnetic compass. Without reference stars or a magnetic field, it would be difficult to determine absolute position on long voyages. As such, if your world only has one visible star, and that star is only visible in the northern hemisphere, oceanic navigation in the southern hemisphere would be risky at best.

However, ships at this time did navigate by measuring current and wind speed, and calculating true velocity from there. The slight error associated with these methods would be acceptable when traveling from port to port, but would compound unacceptably over longer distances, like your version of the trans-atlantic voyage. Thus, you could create a series of small islands, or other recognizable landmarks (watermarks?), in oceans you want transversable. Maybe some areas are constantly rainy, or have strange currents. The landmarks would allow sailors to re-calibrate their relative position measurements, and navigate effectively.

Answer 7

I'd like to point out that the Northwest Passage has in fact been non-navigable until very recently, which affects global comerce and history, as ships have to go a longer route instead.

The availability of "other routes" is another issue with your premise. The non-navigability of some region doesn't affect all approaches to your forbidden contenent.

Another idea for you is the phenomena of rogue waves the specific geology and weather patterns could make them more prevalent in some region. You could also have things like hurricanes that are always in the way.

Answer 8

The question really relates to why the Indian Ocean and such waters was travelled before the Atlantic and the Pacific, as no ocean is actually impassable. The answer lies in the technology being used to navigate the waters. In early marine travel ships would stay close to land for the following reasons 1) Poor ability to sail into the wind, and the risk of becoming becalmed. Thus there existed the real possibility of becoming marooned in the middle of an ocean unable to return to land. This fear was particularly heightened by the poor understanding of how winds were generated (usually attributed to gods) 2) The need to seek shelter in storms. Early boats were not capable of withstanding severe storms and required to be sheltered in severe weather. 3) Limited navigation - limited ability to tell direction, no ability to tell longitude. Travelling in unknown waters far from land meant having no idea where you were, and almost no idea how to get back.

The Indian Ocean is thus easy to navigate by staying close to the shoreline from Africa to Indonesia, and thus this is how it was done. Australia was visited only accidentally through being lost.

An Atlantic crossing requires a leap of faith in very risky waters. Columbus achieved it through luck, and a combination of delusion and information gathered from the fishing community. It is possible the Portuguese fishermen where regularly travelling to the West Atlantic. This knowledge being built up over centuries of experience and the quest for wealth. The Vikings achieved it by travelling in small steps across the far north. This involved expertise in travelling through such waters that only the Vikings possessed, and reliance on rowing as well as sail.

The North Pacific was not crossed as there was no corresponding culture to the Vikings capable of cold sea navigation. The central Pacific was navigated, however, by the Polynesians. This was done gradually over centuries, driven by population pressures, by people expert in navigating such areas, capable of living off the sea, and by using rowing as the primary means of locomotion. It was done in a step-wise manner, from island to island looking for tell-tale cloud formations as navigation. There was likely significant loss of life in such travels.

So the navigability of the world's oceans is an expression of the culture and technology of the people travelling, with the short answer that it is relative easy to travel along the coast in warm climates.

Answer 9

Fast continental drift could create a line of very active volcanoes across the entire seabed. The heat, noxious fumes, completely opaque steam and possibly extreme storms would make it difficult to navigate with anything less than space-age technology - an airtight vessel, air recycling, and some combination of a very fast engine, extreme thermal insulation, and cooling to keep the temperature bearable for the puny humans. Alternatively, use long-range hydraulics and telephony for the first uncrewed mission to Manhattan.

Answer 10

Thick plant-life coverage, sargassum on steroids. Too much drag to go through on sail power...

... until some genius figures out that they contain an oil that can be extracted on the spot and used in the boiler of a steam engine, so ships are essentially eating their way through the algae layer (with a chainsaw on the bow and scoops on the sides).

Some centuries later, the ocean has become empty, and the Age of Sail commences.

Answer 11

• As a couple of answers have already said: because sea monsters.

• There is a society (country) on the other side of the pond (i.e., in your example, in the Americas).  They are very secretive — they have made no attempt to communicate with outsiders, and they really don't like to be discovered.  So much so that any ships that make it to their shores are destroyed, or at least captured and not allowed to depart.

  If your explorers have world-wide communications (i.e., the ability to contact home base by radio and report what's happening), then suppose that the secretive country is so technologically advanced that they have submarines, stealth aircraft, drones, cruise missiles, lasers and/or satellite-based weapon systems, and they destroy all approaching foreign vessels before they know that they're under attack.

• Combine the above and you get merpeople, who live underwater and perceive ships as trespassers.  When they detect approaching foreign vessels, they destroy them (either killing or assimilating the crew).

• A slightly more mundane possibility is maelstroms.  These are whirlpools/vortices in the open sea/ocean.  It looks like these are comparable to underwater tornados, and can be very destructive.  It's not clear how these could make an entire ocean impassable, but maybe anomalies in the ocean floor (underwater volcanos?) could create a chain of maelstroms that would be difficult to navigate without overhead imagery.

  I thought I remembered reading about such a maelstrom (maybe 10 meters in diameter) in open or at least semi-open water (e.g., a bay, harbor, channel or strait).  I cannot now find the page I remember; in particular, I cannot find any very impressive video on YouTube.  Most of the videos I found showed whirlpools that were

  • small (approximately 1 meter) or indeterminate size (no reference scale),
  • in closed bodies of waters (e.g., lakes), and/or
  • manmade!

  This, this and this were the best videos I could find.

• In the course of researching the above, I came across references to vents in the ocean floor spewing sulfur-laden gasses, and underwater volcanos similarly emitting toxic fumes.  If there were enough of these (e.g., a chain of volcanos) close enough to the surface that the gas could reach the surface without being absorbed by the water, they could produce a region of poisoned air that would be invisible.  Sailors wouldn't know what had hit them until it was too late.  But I see other people have already discussed this notion.

Answer 12

Disease. There's a chain of islands across the ocean, they are inhabited by seagoing birds. (The birds only live there because of their nesting habits, they can't survive where ground predators are a threat.) The birds harbor a disease that's extremely lethal to humans.

Rather than be scared of moving things (predators) the birds are scared of large land masses where predators live. Thus the birds have no problem with landing on a passing ship. While it's probably not 100% lethal the survivors can't operate the ship and the ship very well might founder while people are sick even if they would later recover.

Answer 13

I was going to mention something similar to a previous answer. Large aggressive sea creatures like giant squid, prehistoric predators and the like would cause issues. After all, crazy monsters were drawn on early maps!

Maybe, too, shipbuilding technology could be limited. Weak or sick trees would supply poor timber, and without flax or similar there would be no sails. An outbreak of a virulent disease or fungus could destroy floral life.

Answer 14

Rogue waves (also known as freak waves, monster waves, episodic waves, killer waves, extreme waves, and abnormal waves) are large, unexpected and suddenly appearing surface waves that can be extremely dangerous, even to large ships such as modern ocean liners.

On earth such rogue waves present considerable danger for several reasons: they are rare, unpredictable, may appear suddenly or without warning, and can impact with tremendous force.

A 12-metre (39 ft) wave in the usual "linear" model would have a breaking force of 6 metric tons per square metre [t/m2] (8.5 psi). Although modern ships are designed to tolerate a breaking wave of 15 t/m2 (21 psi), a rogue wave can dwarf both of these figures with a breaking force of 100 t/m2 (140 psi).

In oceanography, rogue waves are more precisely defined as waves whose height is more than twice the significant wave height (Hs or SWH), which is itself defined as the mean of the largest third of waves in a wave record. Therefore, rogue waves are not necessarily the biggest waves found on the water; they are, rather, unusually large waves for a given sea state.

Rogue waves seem not to have a single distinct cause, but occur where physical factors such as high winds and strong currents cause waves to merge to create a single exceptionally large wave.

Once considered mythical and lacking hard evidence for their existence, rogue waves are now proven to exist and known to be a natural ocean phenomenon. Eyewitness accounts from mariners and damage inflicted on ships have long suggested they occurred. The first scientific evidence of the existence of rogue waves came with the recording of a rogue wave by the Gorm platform in the central North Sea in 1984.

A stand-out wave was detected with a wave height of 11 metres (36 ft) in a relatively low sea state. However, the wave that caught the attention of the scientific community was the digital measurement of the "Draupner wave", a rogue wave at the Draupner platform in the North Sea on January 1, 1995, with a maximum wave height of 25.6 metres (84 ft) (peak elevation of 18.5 metres [61 ft]). During that event, minor damage was also inflicted on the platform, far above sea level, confirming that the reading was valid.

Thus, if rogue waves were to occur more commonly in certain oceans rather than others, they scientifically would make it possible that certain large bodies of water would be impassable while others that seem very similar would be able to be navigated.

Additionally, since the research is on going on modern earth, their cause could reasonably remain a bit of a natural mystery with several contradictory theories which is logical since they has not been fully solved, nor completely understood our own modern scientific community.

Answer 15

Water density/temperature.

Warmer water has a lower density than colder water. Salt water has a different density to fresh.

This is a real world problem, which is why ships have the plimsoll line that indicates differences between climates.

Now imagine if no one figured this out. (maybe for the sake of drama, you could increase the margin of error). They'd set sail, hit some warmer and lower density water and - because they're heavily laden for exploration - sink.

Answer 16

I would go with a round continent like Antarctica which is so situated that it develops a similar Antarctic Circumpolar Current. Because it never plows into a continent, its speed is phenomenal. It's that current rising over sea mounts at the tips of South America and South Africa that makes those capes so challenging.

It wouldn't have to be at the poles, a circular continent at the equator would develop a circular current from the current on one side flowing north and the the current on the other side flowing south, Just like the currents along the Americas or Australia. It's not inconceivable that such a current could actually deflect ships with pre-1700 analog tech completely around the continent without the mariners being aware of it. Certainly if they hit it from the north or south, they wouldn't have the ability to measure longitude and so wouldn't know they were being deflected East or West.

A semi-sunken continent like North America in the Permian Period (see the area in the Northwest of the map) with a vast shallow continental shelf could form vast coral reefs much like those South of Java and around Australia that could be hundreds of miles across. Such reef structures lay less than 2-3 meters under the surface most of the time and form vast mazes of barely navigable water channels. Add in strong tides (a large moon would help) changing water levels, plus the powerful circulating current and you've got a pretty efficient ship shredder. All the more so because they severely restrict the ability of sailing ships to tack enough to make much headway.

The reef system could also effectively hide the continent by trapping debris washed into the sea e.g. tree limbs such that even if a ship did find the reefs, they would see evidence of a larger land mass.

Sea mounts could deflect the current in spots towards the reefs as well. Likely, the trade winds would as well. With the right combination of tides, currents and storm surge, a ship could sail into the heart of the reef and find itself trapped when the water levels dropped again.

If the continent is tropical to temperate, you would hurricanes and tropical storms several months of the year which would pile up huge waves when they blew against the circulating current. Ships would stall out just as they do on the southern capes with the wind blowing them one way and the current going the other, so the ships just stay in the same place and get hammered to pieces.

Or until they're blown up onto the reefs.

Not until the mariners developed the clocks needed to measure longitude could they reliably figure out the proper way to approach the continent.

Answer 17

Why navigate the Atlantic?

The reason was economic. Before the 1200's, almost anything in the Asia's far East (i.e. after Persia) was a complete mystery to Europeans. There were just a very few vague, scarce and sparse information, rumors and legends. That, of course, until Marco Polo actually traveled to there* departing from Italy, getting back alive (24 years later - it was a looooooong journey), and being able to tell his memories to a friend who decided to write'em down in a book and publish it.

_{* - Well, he claimed that he went there, but there is some doubt and debate about that being or not true.}

Marco Polo described a rich Empire and culture in the far, far away East, called Cathay (which is China). He also described what he knew about central Asia, India, Mongolia, Cipango (Japan) and Burma (present day Myanmar). Goods were already traded indirectly between Europe and China, slowly passing through a long chain of intermediary merchants from many different countries, cultures and languages, such as byzantines, turks, berbers, egyptians, arabians, armenians, parthisans, persians, indians, tibetans, mongols and chineses. Of course, with that such long chain, any goods coming from India or China had a very high price when it eventually reached Europe some years later, because all of the intermediaries would profit from it.

Merchants in the Europe, knew that the goods coming from China and India (specially China's silk and India's spice) accumulated a very high price in the way, so if someone could somehow go there and bring back goods without relying in so many intermediaries, they could sell them at lower prices and higher profits.

With the fall of Constantinople, in 1453, the trade got harder. This happened because the Muslim cultures that dominated Asia's West, Central Asia and North Africa were very hostile to the Christian cultures in Europe (many of them, catholic crusaders). With the fall of Constantinople to the Muslim Ottomans, the situation just got worse, since the last remaining land route between Europe and Asia's far East was closed, without any sea route being known. The result was that India's and China's goods which already were rare and expensive in Europe, became still more rare and expensive.

So, Europeans had very good reasons to try to reach India and China through the seas. Anybody who could manage to do that, would open a highly profitable trade route with no, or perhaps just a very few, intermediaries.

Which countries sailed or not sailed and why?

Opening new sea routes in the Atlantic to the far East would be very expensive on itself and demand a level of organization and financing that only kings could have. Which states could provide that?

• The Holy Roman Empire was a mess, with all of his internal states struggling and warring under a puppet Emperor which actually lacked power.

• Italy was the source of a lot of skilled sailors, but was a mix of small disputing states and city-states, so no state could finance such thing.

• The Pope already had enough problems to worry about, including Muslims and heretic Christians.

• East European states lacked enough organization and navigation skills, and some of them already had problems with the Ottomans.

• England and France were warring during most of the XV century, wars which included Flanders, Burgundy and Austria, so all of them had too many domestic problems to worry about. Also, they already had fairly strong economies, so investing resources in navigating the Atlantic to reach India would be a silly way to waste money that they could instead invest in something else much more economically plausible.

• Denmark, scandinavian states and Iceland were just too far away in the North to have any interest in navigating warmer waters.

• The Muslims were not interested in sea trade through the Atlantic, because they already effectively traded by land or by navigating the Mediterrenean Sea or the Indic Ocean, and they had no interest in trading with European northern states. So, navigating the Atlantic would be pointless for them.

• Far East cultures had no reasons to try to navigate the seas to reach Europe. It was damn too far away, and they already had many people to trade nearby either by land or sea. Also they knew almost nothing about Europe, and most of what they actually knew, was obtained from Persian and Arabic sources, which depicted Christian Europe very negatively.

• Sub-saharian cultures were far too primitive and underdeveloped to think anything about navigating the Atlantic. The american indians also were far too primitive and had no reason to cross the Atlantic in the other way and reach Africa or Europe.

• This leaves only two suitable countries: Portugal and Spain.

During the XV century, Portugal and Spain figthed the muslim kingdom of Granada during the Reconquista war. Portugal finished its part of the war first, so they got a head start. With the war end, Portugal needed some sort of economic activity which would ensure high profit for them, and they already were skilled sailors.

Portugal started to explore Africa's coast in 1418. Their sailing skills leaded to the invention of the caravel, a type of seaship who could sail further and faster than the other existent seaships at the time and could even sail against the wind.

Spain was decided to not be kept behind Portugal, so they started to quickly develop their navigation. However, they were still struggling in war against the moorish kingdom Granada.

Crossing the Atlantic

And then, in 1486, a man called Cristoforo Colombo met the Spanish king and the Spanish queen with a crazy lunatic idea: he wanted to reach Indies by sailing west around the globe instead of sailing around Africa.

The Atlantic Ocean was a formidable obstacle back in the XV century. People already knew that the world was a spheric body, but Columbus believed that the distance between the Canary Islands to the Japan, by sailing west, should be roughly 3700 km, but in fact it is 12500 km - i.e. he severely underestimated the size of the globe. Also, he thought that Japan should be nearer to the equator and be as large as China.

Most "main-stream" navigators would not venture in such voyage because they (rightly) thought that Earth was much larger than that, so that such voyage would be a crazy idea - and they would be right if there was no American Continent in the way and the Atlantic and Pacific Oceans just formed a very large ocean.

Columbus already talked to Portugal's king in 1485, but his idea was rejected as being unrealistic, since the king's experts asserted that the Earth should be much larger than what Colombo calculated (and it is).

Spain also initially rejected Columbus's idea in 1486, but the king decided to keep he around to avoid him going away and telling his idea to someone else. Also, in 1488, Portugal reached the Cape of Good Hope and was definitely not interested anymore in trying to sail west. Portugal was dedicated into the task of reaching India by circumnavigating Africa and nobody else was seriously competing with them yet.

In January of 1492, Spain could finally dismiss Granada once and for all, and then the king could finally dedicate all the efforts into navigation. Columbus continually kept lobbying the Spanish court for all those years, and when he was just finally giving up and leaving, in April, the spanish king decided to give Columbus' idea a try. It was really a very far-fetched crazy idea, but if he was correct, Spain would quickly profit and be able to compete with Portugal, and since Spain's king knew that Columbus was a very skilled sailor, this could not be as crazy as it seemed to be afterall.

So, only seven months after expelling the Moors, Spain sent Columbus in his sea journey to the west. He departed from Spain in August, and from the Canary Islands (the last then-know piece of land in his journey), in September. After five weeks sailing unknown waters to the west, he landed in the other side of the ocean visiting islands that today are parts of Bahamas, Cuba and Haiti.

Portugal reached India in 1498, but landed in South America in 1500 and also started to explore the american continent. France, England and Flanders (Netherlands) joined the race somewhat late to the party.

Ok, how to make that not happen?

There are many points in the history around the last years of the XV century that if just a few things were different, the american continent would be only discovered much later, possibly well into the 1700's, possibly even after Australia's discovery. In fact, its discovery is much more a product of random luck and fortuitous economic situation than anything else.

• If Portugal reached India before Columbus reached America, it is possible that no one (including Spain monarchs) would seriously believe the possibility of navigating to the west far away enough to reach land for some long time. Specially since they already knew at the time that Earth was too large for that being viable. Putting all the efforts into navigating around Africa would be economically safer.

• What would happen if Columbus' caravel were hit by a hurricane and perished to never be seen again? Or if they all were just killed by american indians?

• What if Spain monarchs just dismissed Columbus the same way as Portugal did?

• What if Columbus never born to start with?

• What if Portugal and Spain had some other economic interest (and perhaps didn't invented the caravel afterall)? Or if Granada resisted harder? Or if the Spanish monarchs just suddenly died in an accident and a succession dispute began?

• What if the Ottomans decided to earn gold by seriously trading with some European nations instead of keeping ressenting anti-crusader feelings?

• What if Egypt decided to earn gold by routing India's goods from the Red Sea to the Mediterranean Sea, making both the ideas of sailing west the Atlantic or around Africa too costly, too dangerous and too time-consuming?

• What if Marco Polo never came back to Europe?

• What if Constantinople resisted longer?

With that, the reason to not cross the Atlantic would be just: "Because it is too dangerous, too expensive and is very unlikely to be worth anything. Further, nobody ever heard of anything beyond those seas and have no reason to believe that India or Japan are anywhere near in that direction, so traveling that way is absolutely pointless."

Note that those changes are very simple, plausible and mundane, so there is no need to have supersized shipworms, nor dragons or sensational creatures, nor crazy climatic phenomena, nor crazy geographic or geologic features, nor strange geomagnetic or astronomic phenomena, nor specially unfavourable and unluckily positioned wind, sea-currents patterns or reefs, nor anything severely different than what history was until that point.

What about crossing the Pacific instead?

As soon as the news about crossing the Atlantic and what was found there spread into Europe, cartographers started to debate about what they actually were, and concluded correctly in the first years of the XVI century that those new lands could not be by any means parts of Asia, and they were in fact an entire new continent.

What made the Pacific be eventually crossed is the fact that the discovery of the Americas sparked the interest to actually circumnavigate the globe and more importantly, perhaps discover some other unknown continents or profitable trade routes.

The first circumnavigation of the globe was completed in 1522, after a long and dramatic 3-years journey which killed most of the crew and left some of them in the way, with just a few survivors with bad health being able to complete it (18 out of 237). Only 30 years before (in 1492), the idea of sailing west the Atlantic were considered lunatic, but now the globe could be circumnavigated exactly in that way.

If nobody crossed the Atlantic, crossing the Pacific sailing westward would have no sense. Crossing it from Asia eastward, was already pointless: "There is nothing valuable out there, just a few small and sparse worthless islands inhabited by primitive people". So even if a few people from polynesian islands actually reached the american continent and could come back to their homes, they probably won't be able to spread the news efficiently, and in the best possibility, it eventually would be just another strange fantasy legend in the folklore of a primitive tribe from a remote island that nobody in the civilized world would care to hear about.

So, with the discovery of the Americas, sailors started to fearless navigate into the open ocean instead of just navigating around known coasts. With that, many remote islands around the world were discovered and charted, and it is unlikely that they would be anytime soon if the discovery of the Americas was further delayed.

Answer 18

One more for the list - high energy particles.

The earths magnetic field precesses and occasionally (in geological time) "flips". That means for long periods of time (by human standards) the magnetic poles may be under any given large area of ocean.

(This wouldn't have to be close to a frozen polar area as it is on earth. For example, the poles might not freeze on this planet due to currents or global temperature, the axis of rotation may be greatly inclined, or the magnetic axis is different from the rotational axis).

So suppose a strong, not weak, magnetic field.

You now have your magnetic pole under some specific large area of the ocean for a few tens or hundred thousand years. Unfortunately this area is naturally where the ships would navigate. The stellar environment includes fierce radiation of charged particles which is somewhat focussed and drawn to the surface over a wide area, rather than diverted by this pole, and... voila, lethal ocean passage, if you happen to need a few months to cross it.

The effect would be increased if either, your species is quite vulnerable, or other mechanisms prevent things like ozone or whatever else protects us humans. Perhaps some aquatic life has evolved to produce ozone-destroying gases in modest concentration as well (perhaps because they photosynthesise or use the radiation, or it kills their parasites or at highish levels triggers mating behaviour since this is when food will exist as microorganisms take up the high energy levels, so they evolved this over time as a way to increase their energy source/intake).

Answer 19

Ice! If there was an ice shelf surrounding a Continent like at the poles. The pack ice over North America has only just unfrozen enough in the last few decades to allow shipping traffic. For a long time everyone was looking for the fabled North West passage.

Answer 20

Late to the party:

Make them impassable due to surprising consequences of active volcanic activity. Vigorous bubble formation lowers the sea density to sink boats (especially if "roiling" is a word you like to use to describe seas) and/or limnic (poisonous gas) eruptions would kill entire crews (and render the local sealife unsafe to eat). Decide the midatlantic ridge and other ridges have been actively spewing megatons of sulfur dioxide ($\mathrm{SO}_2$), hydrogen chloride (HCl), and hydrogen fluoride (HF).

Answer 21

We have walked on the moon because we had the motivation to do so and the technology to transport people through a hostile environment with sufficient resources to provide food and comfort for their journey.

Possible reasons are therefore

A lack of motivation. The Dutch visited the West Coast of Australia, Tasmania and even New Zealand long before the English but had no desire to visit regularly or settle here.

A lack of technology (either ship building or navigational), e.g. unable to build a ship big enough to contain sufficient crew to man it and the resources to sustain them, an inability to navigate out of sight of land etc.

The hostility of the environment e.g. prevailing winds and currents, sea worms, coral reefs, availability of food and water along the route.

Good luck!

Answer 22

An underwater landscape with prominent shoals and underwater features that direct the planet's tides in powerful and unpredictable ways.

Most of the world's oceans are fairly deep, in the high hundreds to low thousands of feet. This depth acts as a buffer to the movement of surface layers due to weather and tides; there is so much water under what's being moved around by the moon and winds that in the absence of major, acute disturbing forces like a storm front, the energy dissipates fairly quickly into relatively calm rollers.

However, what if the Atlantic Ocean had significant regions that were shallow enough to have little or nothing below the "thermocline" of water actively warmed by the sun? First off, the aggregate energy in the water would be much higher, because the average ocean temperature would be warmer. Second, the volume of water in the oceans would be much less, and therefore tidal movements caused by the moon's gravitational pull would be relatively major, causing entire land masses to appear and disappear under the water. Finally, irregularities in the ocean floor, because they're in the tidal zones, would cause much more treacherous currents and waves that could swamp or even break ships caught in them at the wrong time. A large enough swath of this kind of water, and it might be impractical to try to navigate it.

The extreme case of such a planet is shown in Interstellar; the first planet, closest to the black hole Gargantua, is a water world dominated by a shallow sea, only knee to hip deep... except for massive thousand-foot waves scouring the surface, generated by the black hole's massive pull. Nothing in our own Earth's neighborhood has that kind of effect on the world's oceans, but maybe if Jupiter were a little closer, and Earth orbited Jupiter closely enough that its gravity were more powerful than the moon's (our own moon can't get too much closer or be too much bigger without gravitational tides tearing it apart), then we might see tides too strong to be navigable anywhere over a continental shelf. A shallow ocean over a very tectonically fragmented floor would produce a labyrinth of volcanic ridges, sandbars and reefs that would keep pace with erosion and present a major navigational hazard to any ship large enough to carry the necessary crew and supplies to traverse that great a distance.

Answer 23

Sea monsters are a good possibility in deep oceans. Some of the planet's oceans are very deep and very wide. And some are shallow and narrow.

Huge and highly intelligent beings live in the deep oceans because they dive very deep to feed like sperm whales. Like sperm whales, they are rarely found in shallow continental shelves.

Some time before a society of fishermen and whalers expanded along all the coasts of the oceans, setting up villages on islands. They rapidly killed off all the shallow water whales. And then, where the continental shelf was narrow, they found a deep water island to use as a base to hunt deep water whales.

Unfortunately for them, some of the deep water whales they preyed on were the sperm whale equivalents, as intelligent as humans and with human like language ability to talk about this new threat and discuss it and pass the news all around the world from one deep water ocean to the next.

And eventually the sperm whale equivalents all around the world made the decision to strike back. The central council order was given to attack all ships where ever they were found. So they smashed the whaling ships from the whaling island and sank them. The islanders soon gave up whaling and restricted themselves to fishing the shallow waters near the island and even raising crops on land.

Meanwhile coastal traders were building bigger and better ships to sail farther and farther. Instead of goods being shipped in short trips and passed from one trader to the next, they started making long coastal voyages for thousands of miles.

Eventually some merchants noted that the charts showed that many voyages went the long way around the coasts of the continents and that sailing straight across the deep oceans would shorten the trips unless they found unknown continents. So exploring expeditions were sent out to cross the deep oceans directly.

And after the expeditions passed beyond the continental shelves they were soon detected by some of the very numerous sperm whale equivalents and attacked and sent to the bottom of the sea. And so all the expeditions sent to cross the deep oceans disappeared without a trace or any clue to what happened to them.

And so the sailors of that world always sail along the coasts and go the long way around the borders of continents and never know what islands or continents may be in the middle of the deep water oceans. At least until the sperm whale equivalents eventually forget about being hunted and the order to attack all ships, or until the land people develop iron hulled steamships capable of resisting attacks by the sperm whale equivalents.