This is one of a series of questions centered around how an isolated group of people would survive. Each question focuses on a single aspect of survival. Details about the peoples' situation are below:

In a novel I am developing, a village's worth of people is living on a peninsula. The isthmus connecting the peninsula to the mainland is very narrow, and spanned by a wall, which prevents the people from leaving (there are deterrents preventing them from climbing the wall or otherwise circumventing it). They also cannot swim around the wall. This also means that no land-based animals can cross onto the peninsual from the mainland. The inhabitants have to live with what they have. For the sake of details, assume the peninusla is roughly the size, shape, and location of Mahia Peninsula.

This particular question deals with water being an escape route. For the moment, assume that there is no way for the inhabitants of the peninsula to approach or get across/around/under the wall. The wall is not an option. Flight has not yet been dreamed of (think 13th century). The only remaining option is the sea.

It is very important that the inhabitants have no way off the peninsula. This is vital to the novel. The sea cannot be an option for escape, and there can be no doubt about this. What can I do to make sure escape via water is completely out of the question?

For technology purposes, assume the inhabitants of the village are peasants of the medieval era, perfectly adept at building.

I've marked the answer by Lu22 because I feels it provides the most air-tight deterrent. Everything else, while great, still has the problem that given enough attempts, someone is bound to succeed in the end (granted no one will know that he succeeded, but still).

I would like to mention that the answer here has a convenient index of sorts of everything listed. The answer provided by Strongo was also a very helpful one, and combined with sea monsters I think would be an awesome combination.

  • $\begingroup$ That's pretty hard considering if it is a fishing village! Alright now I can see the problem... how about undersea volcano throwing up deadly gas bubbles. $\endgroup$
    – user6760
    Nov 18, 2016 at 6:45
  • $\begingroup$ For the sake of the question, can we treat the peninsula as effectively an island? Also are cultural/religious reasons ok? And does it have to be impossible to sail off the peninsula, or just extremely difficult/dangerous? $\endgroup$
    – Guran
    Nov 18, 2016 at 6:56
  • $\begingroup$ Is your story based on earth, or is it possibly on an alien or fantasy world? A non-earth location allows for an alien species or some magical danger/barrier in addition to normal earth-type limits to using the sea to circumvent the wall. $\endgroup$ Nov 18, 2016 at 6:58
  • 2
    $\begingroup$ This sounds a bit like the set-up for Destiny's Road by Larry Niven. $\endgroup$ Nov 18, 2016 at 12:21
  • 1
    $\begingroup$ M. Night Shyamalan's The Village dealt with this very issue. He solved it by a circle of elders creating the illusion of scary monsters living in the surrounding woods. $\endgroup$
    – Tony Ennis
    Nov 19, 2016 at 4:01

17 Answers 17


There's a whole other route you can go.


Whoever built the wall did so in order to keep the people in. So how about as an extra measure they had the entire village addicted to a substance only found on the peninsula?

For example a fungus or plant that only grows there under strict conditions, or even a unique tree's (that cannot be propagated) sap.

If not consumed daily the villager suffers extreme withdrawal symptoms or even death. The addiction could be passed from mother to child via breastfeeding, so the villagers can't phase it out.

Alternately it could be a disease affecting everyone (such as HIV), with the substance delaying its potency and allowing a normal life if consumed regularly.

In order to keep the people from hoarding it and being able to travel, the substance would need a very short shelf life.

This would keep the people in the peninsula even without the wall.

  • $\begingroup$ Interesting take on it, Lu22. Thank you. This might not be the exact route I would take, but it's given me some interesting ideas. $\endgroup$ Nov 18, 2016 at 17:48
  • $\begingroup$ An mentioned in an earlier comment Destiny's Road by Larry Niven has a lot of interesting thoughts on this problem. There is also a diet dependency though it is used to control the inhabitants and actually comes from the mainland. I recommend this book a lot (there was a companion book The Legacy of Heorot that was also a good read). $\endgroup$
    – KalleMP
    Nov 19, 2016 at 20:30
  • $\begingroup$ Glad I could help.Would like to see the final result. $\endgroup$
    – Lu22
    Nov 20, 2016 at 14:05
  • $\begingroup$ "A poison — so subtle, so insidious . . . so irreversible. It won′t even kill you unless you stop taking it." (Dune) $\endgroup$ Nov 21, 2016 at 16:45
  • $\begingroup$ I didn't even think of Spice. Yeah that's pretty much on par. $\endgroup$
    – Lu22
    Nov 21, 2016 at 17:03

Give them nothing to build a boat out of matched to the nastiest coastline you ever did see.

No wood, no reeds, no beach, just jagged cliffs facing onto the Southern Ocean. Much like Zavodovski Island.
Zavodovski Island

House building is able to cope with this restriction, it just makes it a bit harder and more expensive to build, as long as they have the stone, but far from impossible. However by taking this option you're effectively limiting the people to stone age technology unless they already have active mining and iron working before the wall was built.

The problem here is the breadth of stuff you can use to build a seagoing boat.

  • Reeds
  • Timber
  • Light wood and Hides

Which means that to keep the people on the peninsula you need to prevent them from having any wood, any large animals, any large rivers or accessible shoreline. People will build boats, all they need is half a chance. If you make them desperate enough they'll probably work out how to build a boat out of the skin and bones of their dead relatives.

  • $\begingroup$ I wanted to note that doing this wouldn't necessarily limit technology, as you could have bogs from which peat could be harvested & dried then burned. This puts me in mind of st kilda, which aside from a natural harbour is basically unassailable, and high winds prevent tree growth. islanders living there kept cattle and sheep. $\endgroup$
    – jammypeach
    Nov 18, 2016 at 11:46
  • $\begingroup$ @jammypeach, I don't know that peat burns hot enough to smelt and it's really hard to mine with stone tools. If you have surface coal and ores then you're ok, but it's not common to have them close to each other. Of course this is fiction so if that's needed then it can happen. $\endgroup$
    – Separatrix
    Nov 18, 2016 at 11:51
  • $\begingroup$ I'd like to have mining going, as my villagers do need weapons (as in greatsword, not bow & arrow). What would I need to at least get them started in that endeavor? $\endgroup$ Nov 18, 2016 at 18:58
  • $\begingroup$ This is essentially what eventually isolated and nearly killed off the population of Rapa Nui. No large trees to build boats. A generation or two would be all that it takes before the knowledge on how to build boats is completely lost. $\endgroup$
    – Samuel
    Nov 18, 2016 at 22:17

Strong currents moving in wrong direction, extensive coral barrier, wind blows constantly the wrong way, permanent mist, constant heavy surf, etc. There are many shores in the world which are utterly unsuitable for navigation. See for example the Skeleton Coast, https://en.wikipedia.org/wiki/Skeleton_Coast. Then question remains why don't they simply go around the wall...

I guess that it must be important for the plot that the people are on a peninsula, probably because at a certain point some enemy comes over the wall, or one or more characters find a way to cross it. Otherwise you could place them on an island or oasis.

  • $\begingroup$ There is only see around the wall. $\endgroup$ Nov 18, 2016 at 7:47
  • $\begingroup$ Yes but the wall ends in the sea at both ends. They could use small boats or rafts or personal flotation devices and go around unless there are fierce predators in the sea... $\endgroup$
    – AlexP
    Nov 18, 2016 at 7:48
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    $\begingroup$ An oasis is much harder to leave than anywhere with a shoreline. $\endgroup$
    – Separatrix
    Nov 18, 2016 at 7:59

I have yet to see this, so I'll add it as a suggestion: make the sea itself inhospitable to organic materials (including humans).

As in, make it so that the water has some weird chemical composition or there is a microbe/small creature in the water that will eat away and dissolve any organic material in a matter of hours - think barnacles on steroids - far too soon to reach anything beyond the sea and far too late to swim back to shore and tell anybody about the problem.

Now, I don't think you want to kill people who just went for a quick swim, so you could have this toxic element start in the deeper ocean. Basically, you make the area near the shore safe-ish and not have this problem, but if you sail a day or so out then your boat will sink and you will certainly drown or get dissolved by the same effect that destroyed the boat.

13th-century technology is no way capable of making a blue-water vessel out of non-organic materials or of creating a coating which can resist the water's corrosive abilities, so this barrier will completely prevent navigation by water.

You'll just have to make the wall extend 25 miles out to the side from the peninsula, and you'll be good to go.

  • $\begingroup$ Also, in case somebody wants to say that they could make a metal or stone boat - even assuming you could somehow make a boat out of nothing but inorganic materials in the 13th century, would you also think to make the boat enclosed and watertight in all directions? Waves splash, and the water eats people, too. $\endgroup$
    – Jeutnarg
    Nov 18, 2016 at 19:24
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    $\begingroup$ Like the volcanic acid pool in Yellowstone that devoured a guy who fell in recently. $\endgroup$ Nov 18, 2016 at 22:08

Note that you don't need specific or modern materials to make very good boats.

"... the traditional umiak was made from a driftwood or whalebone frame pegged and lashed together, sometimes with antlers or ivory, over which walrus or Bearded seal skins are stretched. Oil, usually from seals would be used to coat and waterproof the seams." - https://en.wikipedia.org/wiki/Umiak

Polynesians travelled thousands of miles across open ocean without access to modern navigation techniques, using dugout outrigger canoes with no metal parts (https://en.wikipedia.org/wiki/Polynesian_navigation).

So from the frozen north to the tropics in all sea conditions, people are very good at seafaring, but this skill develops over many generations. If the people had been recently deported there from a landlocked place with no navigable rivers then they wouldn't have the knowledge or understanding of boats for some time.


Building up on Lu22's answer...

Unavailability of non-perishable food

The scenario does not require that people are not able to build boats, it just requires that people are not able to get to any other place via sea. So, we just need to make sure that the amount of supplies needed to get to any other place is greater than the amount of supplies that can be collected and stored properly in a boat.

This means no preservatives (no salt mines, no salt plains, no desalination technology), no milk-based products, no smoking meats, no nut trees, no edible animals on board, no fishing.

It is not needed that the amount of perishable food is exactly zero. We can assume that the available amount of supplies that can be stored for a long time is small enough that sea travel is only viable for a very small amount of man-days.

e.g. the islanders spent a few years storing supplies for twenty man-days, and a lone explorer was sent away, but came back after three weeks, starving.

Quick-onset scurvy

We all know about scurvy, right? The disease that plagued pirates because they didn't eat enough C-vitamin?

The problem with scurvy is that it takes several months for sailors to display symptoms.

Simply create a scurvy-like disease, but with an onset of days instead of months. The disease is easily cured by eating something perishable (the islanders cannot store more than a few days worth of it in a boat), or by being in a completely dry and warm environment.

e.g. the islanders sent lone explorers out to the sea, but they came back after a few days, disoriented and very sick.

Lack of navigational tools

Get rid of compass technologies (no deposits of magnetite or similar minerals), and prevent star-based navigation (due to weird climate patterns, there's heavy fog at night out in the sea).

Without compasses or Iceland spar, it's pretty much impossible to navigate the sea in a straight line. Watch the few first episodes of Vikings if you need a bit of background on that.

Islanders would still be able to sail around the island, as long as they have a clear view to the landmass. But sail more than 30km away from land, and sailors get lost fast.

Obviously I'm not addressing the issue of sailing around the isthmus wall, but the OP is explicitly handwaving that issue for this question.


Setting aside that thing about the wall, and taking into account the number of people and the era, you might preventing escape by a combination of a huge predator and a lot of superstition.

Over time, someone will ty to escape. That's only human, so assuming they would not seems to be out of the question.

If you have a large reef around your peninsula, this would provide plenty of food for your village, making life easier, and reduce the pressure to migrate. But the same would be true for large predators.
It would also prevent large vessels from escaping.

Sooner or later someone will fall victim to a large predator. If we are talking about sharks, who normally don't feed on people but just attack because they mistook them for something else, you will have badly mainmed boats and corpses on your sore after an unsuccessful attempt of escaping.

Perils tend to become bigger and more dramatic the more they are reported at night around a campfire. After the second or third attempt, your villagers will have interestin stories of enormous monsters lurking in the sea, hunting humans, tearing them apart, and whatnot.
Also, they will be green, tentacled and fanged, of course.

as a result, joined efforts of leaving will be strongly discouraged. And while from time to time a young hotspur will try to escape, mostly failing very painfully, the only attempts that actually stood a chance would never happen.

What is more, even if our young hero managed to escape, the villagers would never know, and would most likely assume that the monster ate him whole.

So, while it is hard to conceive a way of preventing escape by geographical means, human nature might do the trick of locking them in.


What is on the other side of the wall? If there are hostile people or dangerous animals there, then someone who swims or builds a boat to go around the wall would have no safe place to go.

The water could be full of sharks or other dangerous sea creatures to prevent people from swimming.

People could have the engineering skill to build houses and tools but have no idea how to build a boat. But this gets weak if we're talking about simply getting around a wall across an isthmus. I could easily believe that people can't leave an island because they don't know how to build boats capable of travelling hundreds or thousands of miles. Tougher to believe they can't figure out how to build a simple raft that would take them past a wall. How thick can this wall be? A few feet? Strapping a few logs together so you can sail ten feet doesn't seem all that tough, you'd think someone would think of it sooner or later.

How about something cultural? These people have just never thought of the idea of travelling by water. Or they have a superstition about water, they think ghosts inhabit it or something. Seems to me that sooner or later someone would test such beliefs, but it might be plausible.

Can there be people actively patrolling the waters around the peninsula? Powerful warships quickly sink any boat these people can build and massacre the survivors?

Oh, or how about: the shoreline is steep cliffs. Sure, they could build a boat. But then they'd have to lower it 100 feet to the water and climb down. Then they sail around the wall but to get ashore, they would somehow have to climb from the boat up a 100 foot cliff. It would be possible, of course, but very difficult, the few who try it fail and everyone else just accepts it as impossible.

  • $\begingroup$ All good ideas, but like you said, someone would eventually succeed. These people really want to get off the peninsula, so it's plausible that they would keep trying, even ignore their beliefs, to do that. Eventually someone is going to succeed. It needs to be outright impossible for this to happen. $\endgroup$ Nov 18, 2016 at 17:51

Other answers cover a point I was going to make - if you remove any reliable source of wood from the inhabitants, they have nothing with which to construct a viable ship, and so wouldn't be able to go much of anywhere.

You have said in your question that there is no way around the wall, but then gone on to specify the sea as their remaining escape route, so the following works on the assumption that your restriction on their ability to bypass the wall is geared towards land-based methods.

With this in mind, there does remain the possibility that they may be able to cobble together some short-range rafts though to bypass the wall by following the coast to the mainland.

My suggestion to avoid their doing this would be to replace the wall with a mountain range.
Doing so would be able to give rise to incredibly violent storms resulting in any navigation following the coastline to effectively be scuppered by a wall of it's own, in the form of treacherous currents, unpredictable storms, and consistently violent winds.

This could also lead to reefs being present in the area, in the form of the mountain range extending out past the land to either side of the peninsula along the seafloor.
This would provide the dual advantage of further destabilising currents and adding the risk of the sharp rocks in to the equation for someone attempting to traverse the waters in anything from a rickety raft to a full-drafted ship.

  • $\begingroup$ I like your mountain idea, for the storms, reefs, and currents. I don't want the wall to be a mountain (needs to actually be a wall for the novel) but there's nothing saying a mountain range can't be directly behind the wall. Are we talking about big snow-capped mountains or simply large hills here? $\endgroup$ Nov 18, 2016 at 18:03
  • $\begingroup$ @ThomasMyron Well I was thinking enormous snow-capped ones for the sake of replacing the wall as an impenatrable barrier - the bigger and more dramatic they are the more believable the associated effects will be as well. But at the end of the day, it's your world, man :) $\endgroup$
    – Strongo
    Nov 18, 2016 at 20:18

I would do either:

  • A predator in the sea that will devour anyone that crosses its path.
  • Sharp underwater reefs
  • Maelstrom's
  • Currents pulling a ship against the cliffs
  • Alot of fog so nobody can navigate anywhere.
  • Lots of rogue waves
  • Vulcanic or seismic activity underwater which creates alot of waves. (Tsunami)
  • Thunderstorms and bad weather which would be visible from the shore.

The last one is my favorite since it stops people from even thinking of leaving the island.

You could ofcourse combine some ideas to make for a very interesting and dangerous enviroment.

I hope this helped please ask questions or give constructive criticism if you would like to know more or if I missed something.


Why not get psychological? You may have heard of the supposed '5 Monkeys and a Ladder' social experiment. Basically, the original group are punished whenever one attempts something- thus it becomes taboo to do that thing. Finally you end up, a few generations later, with a group that will not do something despite not knowing why. See also: The Village

How to apply this? Your original group of settler, prisoner, whatever, were closely observed. If any tried swimming away then their settlement was raided or someone 'disappeared'. Naturally over time they realised that trying to swim away had a consequence. And the elders, or ruling group now actively prohibit it.

Of course it's not water-tight (ha). But for a bonus there could actually be something in the water as others have suggested- but this isn't ever outright said, just hinted at.


Add a continuous fog bank on the mainland that lifts on very rare occasions.

When visible make the land behind the wall all the way up both coastlines appear lethal with lava, scorpions, suffocating smoke, triffids.

Let the local folklore explain the wall as a safety measure rather than imprisonment. Have the dangers forever creeping around and over the wall requiring vigilance and pruning. Have some of these dangers artificially generated if required.

Add jelly fish, electric eels, piranhas, Candiru :-) , sharks etc to make attempts less tempting, maelstroms, foaming or burning methane.

Also make the shore landing treacherous with venomous sea urchins, jagged sharp cliffs and cannibal tribes.

Let your imagination soar, all you can do is make it hard, the outcast will always find a way. As mentioned in a comment the book Destiny's Road by Larry Niven has a lot of interesting thoughts on this problem. There is also a diet dependency for the population thought the trace element actually comes from the mainland and is traded with the peninsular inhabitants so they are hostages. I recommend this book as a good reference scenario to try and improve on.


Acid leeches.

They live just offshore, feeding on driftwood, beached whales, and the carcasses of fallen sea birds. Nothing else swims in those coastal waters - not for long, anyhow. The leeches' highly developed sense of smell allows them to detect any organic matter - they can eat more or less anything. When they smell something tasty, they'll swarm over it, latch on, and inject a toxic acid from a spine inside their acid-proof suction-cup mouths. The wood - or if they're lucky, meat - dissolves quickly, and they drink it up, swelling up to twenty times their original size - up to the size of a loaf of bread. Afterward, they'll swim along the shoreline, dispersing their eggs over a wide area. There, the adolescents will hatch, fully formed, and sit dormant, for months if need be, until they scent food. They exist along the whole shallow coastline, for miles out, at a density of up to ten per square meter.

One bite is enough to kill a child, or seriously weaken a strong adult. Besides having a tendency to bog down and rapidly consume any wooden boats, the leeches also have the unsettling ability to climb and crawl, like huge, horrifyingly fast alien slugs. Once they've caught the scent, they'll squirm with all their might in pursuit of delicious humans, mindlessly willing to die for the cause. Therefore in addition to dealing with a heavily encumbered, progressively disintegrating ship, any hypothetical sailors would be engaged in a ceaseless battle of whack-a-mole in which one missed strike could result in their death.


if all the mainland coast for many miles is high cliff, they would just need patrolling to keep people on the island. just make sure no one who makes it to the mainland lives long enough to makes it back to the island. the big thing is it may not need to be perfect, just nearly impossible for a few generations, once they believe there is no way off the island they will keep themselves on the island, you don't need to make impossible just too difficult to seem possible.


A small community with humble technology could be thwarted from escape by a combination of factors without having to resort to depriving them of wood.

Imagine strong surf breaking against a rocky cliff of a shore line, making leaving nearly impossible. If our intrepid explorer gets past the surf, the rocks and shoals, then he or she would be swept out to sea. Since they can't see anything but water (which can be accomplished by having no no land or other feature for about 50 miles. The highest point on Mahia is 1302 feet high. A six foot tall person on top of the hill could see a little over 44 miles on a clear day.) there is no obvious destination.

Add to this a dead zone around the peninsula due to Phosphorous(see Dead zones), and any explorer swept out to sea would never make it back alive. In addition, no fish in the water would take away fishing as a motivator for building boats.

There is a reason the wall was built. Whether to keep them in or keep them out, lore would build up around the wall telling of the horrible creatures waiting on the other side.


These are some of your options to make the inhabitants captive of the peninsula:

Fast water currents, as suggested by AlexP

No detail on this, as it has been already covered by AlexP.

Sea Monsters

If your world has magic available, then we are talking about true horrors like krakens, gorgons or prehistoric sea monsters such as pliosaurs, megalodons and tylosaurs.

The very sight of these sea monsters in deep waters would keep the people from any thought of getting into deep waters, although they would be free to go fishing in the shallow shore waters (upto the depths of 50 feet or so).

Lack of required materials for shipbuilding

Building a ship requires metal. If you don't have any metallic ores on the peninsula, your people would not be able to build any ship in the first place. While they would still be able to hollow out canoes, these would not be large enough to stock a weeks' long reserve of food and water, making long sea journeys impossible.

  • $\begingroup$ You can build big boats entirely out of wood and rope, you don't need metal. You use wooden pegs where you're thinking nails are critical. $\endgroup$
    – Separatrix
    Nov 18, 2016 at 8:46
  • $\begingroup$ @Separatrix: Yes, but ... how do you chop and shape wood without metal ...? $\endgroup$ Nov 18, 2016 at 10:57
  • $\begingroup$ Using bits of wood small/light enough that they don't need special tooling besides your hands and maybe some stone tools. $\endgroup$ Nov 18, 2016 at 11:13
  • $\begingroup$ It's harder and slower, but stone tools can do it. You can get a really good edge on flint. $\endgroup$
    – Separatrix
    Nov 18, 2016 at 11:19
  • 1
    $\begingroup$ You can build a quite large catamaran with no metal. Take two 70-foot+ trees, hollow them out to make big canoes (this can be done with fire - not even stone is required) and strap poles across them to frame out a platform. You can then build a little house on the deck for shelter and food storage, put up a sail, and you have a decent-sized, totally seaworthy boat. $\endgroup$ Nov 20, 2016 at 21:50

Ok, best I can come up with would be for the peninsula to have 1) a fairly steep shore/cliffs with the livable land say 20-250 meters above the sea level and 2) be in a crater sea with 3) the isthmus surrounded by that crater sea then by a crater wall 10~20 meters tall with 4) it's connection to the mainland being a (150-200 meters) steep ridge. We could say the unique geography was created by a combination of meteor impact and volcanism. 5) In the crater sea there are many large blue holes letting ocean water easily flow in and out of the crater sea allowing us to have some life in the water. 6) While there are no longer any obvious signs of volcanic activity, CO2 is seeping up and filling the bowl of the crater from the surface of the sea up to say at least 10 meters but below hospitable zone for the peninsula.

This should leave the steep ridge of the isthmus as the only survivable passage to and from the island and give your wall the intended meaning.


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