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I'm trying to write a story about falling off the edge of he earth during a sea voyage and would like to incorporate as much as detailed and accurate physics as possible.

For this to work, I know there needs to be a sudden noticeable change in the curvature of the planet. Is a flat earth possible if, for example the rotational speed is high enough (I don't really think long-term gravitational stability is required)? I know a torus-shaped planet is possible, but I believe that an ocean in a torus wouldn't feel much different.

Another idea I had was to somehow incorporate a black hole, essentially sucking water at the end of the ocean upward and creating an "edge of the world" but I'm not familiar with blackbole mechanixs at all.

I'm really just trying to think of some accurate physical situation I could set up to get the effect of an edge of the ocean where a ship could fall up or down. Any ideas at all would be welcome.

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    $\begingroup$ Diskworld. The rulebook for what you seek is to be found in the Diskworld novels. No, sorry. Neither physics not even the much more relaxed fizzics will suffice, you will have to go full-bore-and-four-elephants wild Magic for this one. If you try to handwave an edge of the world while remaining within the Laws of Nature, you will develop both tennis elbow and carpal tunnel quite promptly. $\endgroup$
    – PcMan
    Dec 23 '20 at 21:43
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    $\begingroup$ Slightly related on the topic - Terry Pratchett wrote a sci-fi novel called Strata. It's before the Discworld series and it's the first time he used a flat world. Although it's in a rather different context - it's not a fantasy take with turtles, gods, and magic. Worth a read, though. $\endgroup$
    – VLAZ
    Dec 23 '20 at 21:45
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    $\begingroup$ If you want to incorporate something resembling realistic physics, you might want to start with some research into real physics then. For instance, research "why are all planets round". $\endgroup$ Dec 23 '20 at 22:08
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    $\begingroup$ Looks like you're going to have to devise your own Physics! $\endgroup$
    – elemtilas
    Dec 24 '20 at 4:46
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    $\begingroup$ I’m voting to close this question because the asker openly admitted this is for a college admissions essay with a question about falling off the edge of the world. $\endgroup$
    – Daniel B
    Dec 24 '20 at 9:53

15 Answers 15

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The problem with a cliff in the ocean a ship might fall over is that the water falls over it as well. That means that your ocean will drain vigorously until it's below the height of the cliff.

You could achieve a similar effect though if you let your ship navigate a lake or wide stream which drains through a gigantic waterfall, like the Victoria or Niagara falls. Imagine a vast flat highland with no easy way down to the lowlands. You'd have quite calm water until near he edge and explorers who never were in this region before might get caught by surprise and end up in a different world.

Edit: So your prompt is for a 13th century sailing vessel. Ok, but it doesn't have to be European, does it? Put it on a "Lost World" style tabletop plateau, the explorers being from a native tribe. Otherwise, it's kind of hard to keep physics real.

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    $\begingroup$ that would have worked but the prompt is literally "You're on a voyage in the thirteenth century, sailing across the tempestuous seas. What if, suddenly, you fell off the edge of the Earth?" $\endgroup$ Dec 23 '20 at 22:08
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    $\begingroup$ If you have a sea that is closed in a ring of ice or stone and then the stone cracks, there would be an edge that a ship could fall over for some amount of time until the water equalized out. $\endgroup$
    – SRM
    Dec 24 '20 at 0:10
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    $\begingroup$ @OVERWOOTCH - It would have been really useful if you had included that little tidbit of information in the question that you asked. Please edit your question to include that important constraint and any others that would help people to help you solve your problem. $\endgroup$
    – Paul Smith
    Dec 24 '20 at 1:23
  • $\begingroup$ "but the prompt is literally.." prompt? sounds very suspiciously like a "do my homework" question also that shoulda been in the question to start with $\endgroup$
    – Topcode
    Dec 24 '20 at 2:57
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    $\begingroup$ "asking for cool situations I could set up to create an "effective" edge of the world" which just so happens to be your homework $\endgroup$
    – Topcode
    Dec 25 '20 at 4:34
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About 5.3 million years ago, the Atlantic Ocean broke through the straits of Gibraltar and flooded the Mediterranean basin. It’s estimated to have taken somewhere between several months and two years, and during that period there was indeed an enormous water flow several miles wide. The linked Wikipedia article sadly says “Studies of the underground structures at the Gibraltar Strait show that the flooding channel descended gradually toward the bottom of the basin rather than forming a steep waterfall”, but you could have a slightly different setup that does form a waterfall.

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You could change the rules of gravity so that gravity pulls in a single specified direction (unlike the real world where it acts between every object with mass and every other object with mass according to an inverse-square law), your world could be functionally identical to the real world in every way for the inhabitants with several major exceptions:

  1. If you go far enough in a given direction and the world isn't infinite, you will find an edge. This provides the basis for a mechanism by which your characters can sail off the edge.
  2. This also means that, unlike on Earth, you can't just pick a direction and wander and eventually end up back where you started. You have to change direction to avoid falling off the edge. Sailors and other travelers on this flat planet would quickly become aware of this problem and be careful to avoid falling off, but storms are known to blow people off course, or if the navigator dies from scurvy or is marooned following a mutiny...
  3. In the real world, tides are caused by the gravitational pull of the moon. If gravity worked differently, there would be no tides unless you came up with an alternative explanation. No tides means no sandy beaches and no tide-pool dwelling creatures

This solution also presents a different problem: How does the water stay on? If the rim of the world is shallow and evaporating water eventually makes it back into the ocean via rain, this would mean that the ocean level rises and falls significantly with major storms, which could replace the tide cycle. This also means that the edge of the world is only submerged following a deluge, and might otherwise be safe to sail near. That means that a sudden storm near the edge while people are sailing would raise the water level enough that water begins pouring over the edge, bringing any nearby ships with it, so experienced sailors would be wise to keep an eye on the sky for dark clouds.

As the author, you can make as many small tweaks to the physics of the world as you need to in order to tell your story. The important thing is to keep the rules consistent throughout the story.

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Check your title - was it "falling off the edge of the Earth", or "falling off the edge of the world". If your requirement is the physics of falling off the edge of the your world and your only constraint is hard physics, then there might be a way... Think of the ring world from Larry Niven novels or the huge torus satellites from the 'Culture' universe of Iain M. Banks, and you have your starting points. You can only fall off the edge if there is an edge, and there is no such edge on the surface of a sphere, therefore you can not use a traditional planet such as the Earth. However, if your "world" is the environment you (or your hero) has evolved in then more options present themselves. You can't use a flat world like a Frisbee because the water has no reason to stay there. A torus shape could work but it would be a struggle to make it make sense. The simplest body of water with an edge (from a physics perspective) is a bucket. A fast growing life form that evolves in a bucket of water and develops intelligence, tools and transport will want to explore its environment, and will, sooner or later, reach the edge and, not unexpectedly, try to find out what is on the other side. The physics of falling off the edge is easy. The physics of learning what is over the edge, and communicating the findings back are intriguing.

To add an entertainment value, consider the fact that laws of physics as measured on the surface of a bucket of water do not have to agree with the laws that exist once you go over the edge. Especially if the bucket is tied to a piece of rope and being swung around the head of a physics student (https://www.physicscentral.com/experiment/physicsathome/centripetal-force.cfm)

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  • $\begingroup$ I think I could actually work with this. I came across the "science of ring world" series. Are they of any use? Or will i have to investigate the mechanics of such a planet on my ow? $\endgroup$ Dec 24 '20 at 4:31
  • $\begingroup$ This reminds me of James Blish's classic story "Surface Tension" where the world of microscopic beings is a pond on an alien planet. They have a space program equivalent to develop water filled vehicles to travel overland to other ponds. On a larger scale human sized beings could livone one of two giagantic "buckets" attached to each other rotating around eachother to similate gravity in the "buckets" o rother containers.. $\endgroup$ Dec 24 '20 at 19:04
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The earth is 'flat', but in four dimensions, allowing a 3d sphere to have 'sharp edges'.

Now that I have written the reply in full and look back at it, I am fairly confident this is a mostly hard science explanation of the edge of the world and is handwaveably consistent with physics. At the very least, it doesn't invoke alterations to gravity, require a passing rogue planet, glitches in the matrix (still pretty cool) and only relies on conventional tidal forces and rotation except working within the framework of 4d space, which itself isn't physics breaking.

[Preface about 4d objects. Optionally skip-able]

To first get the average reader acquainted with 4d objects, this is the most intuitive way I have seen to explain them: https://4dtoys.com/ The next most intuitive way I can explain them is, when you are seeing a 4d object in 3d space, you are seeing a 3d slice of the 4d object. It is the same concept as a using a series of 2d slices to recreate a 3d object, such as with an MRI where the brain can be visually analyzed with 3 moving 2d planes which display an image of the brain at any given x,y,z.

Here are some examples of 4d objects intersecting with 3d space:

enter image description here enter image description here

Now, imaging a 4d sphere moving through 4d space so that it passes through our 3d plane. An observing person would first see a small ball appear floating some distance off the ground, possibly rolling around, and it mysteriously grows until it touches the ground, and then starts to shrink until it disappears from view when it finishes its intersection with our 3d plane.

[End preface on 4d objects]

Conceptually, Earth could be a 4 dimensional hypersphere, where each 3D slice of space has its own continents, plants, people, and civilizations which have 'slipped off the edge of the world'. Like a 3d sphere, a 4d sphere can rotate along the fourth or 'w' axis and likewise have 4 dimensional perturbations of tectonic plates.

Sometimes, there are earthquakes, storms, or other large scale events which can displace mass from one three dimensional slice to another. Another option is that as the Earth rotates on its 4d axis, sometimes a large amount of mass of land or ocean may find itself in a position of high potential energy should it move on the 4th dimentional axis. Hence, it does, displacing a large amount of ocean/land. In either case, to an observer, this mass would mysteriously disappear or appear based upon which direction it is moving relative to you along the w-axis.

Now, for an example. You are a sailor on the ocean. It's midday, the skies are clear, and you are looking at a ship on the horizon. Then, it suddenly plumets into the ocean as if in freefall. As an uneducated sailor, what would you think happened? Obviously it fell off the edge of the world. Now, as someone partaking in this thought experiment, answer what happened? A tectonic plate shifted, displacing its energy along the fourth dimensional w-axis, heaving a large amount of ocean upwards along the w-axis as well. In this particular slice of 3d space, it just so happened to knock a bowl of ocean out from under the ship, sending the ship and its sailors plunging a thousand feet into a vacuum to be engulfed by the ocean as it fills back in. This could be accompanied by a very loud sound (the air rushing in to fill the void) and possible atmospheric effects (hence a green flash). This, however, is more of a maelstrom event but it serves as an illustration.

Now, lets move onto an actual edge of the earth type event. Imagine instead of a perfect hypersphere, the Earth, like in 3d, in 4d has ridges, mountains, craters, and other terrain features on four dimensions. As it rotates, tidal, gravitational, and inertial forces may draw water over such a ridge. To an observer in a 3d plane, it would appear as if the water, once standing still, would start rushing towards a place where it disappears. As it would take a certain threshold of gravitational forces to achieve this, just like pulling water over the lip of a cup, it would be sudden and all at once. Also, it could extend all the way to the sea floor and be many dozens or even hundreds of miles in size. This would be large enough that it would, in fact, appear to be the edge of the world and might fool even a modern sailor.

This phenomenon will be temporary, lasting only so long as the Earth is rotated in the right direction for the water to 'slosh' over the edge, not that it could drain the oceans anyway. The 4D Earth system will conserve the same amount of water, it's just in a different slice and it will eventually slosh back into this slice when the right conditions allow. Who knows, maybe there's a whole water cycle happening across all 3d slices, where moisture rich air gets blown across slices hence why we still have trouble predicting weather, but I digress.

This kind of edge of the world event would extend horizon to horizon (since mountain ranges can be ludicrously huge) and, while it would be temporary, you might be able to predict it with the right understanding of the 4d geography and eventually figure out how to traverse the falls. Now, a ship plummeting over the edge of could likely end up traveling to a different 3D slice, as there are 4 dimensional forces at play displacing the water and them along with it. Once there, they will remain until they find another edge of world style event, as there are no more 4d forces at play.

Each slice could potentially have its own ecosystem and civilizations formed by people 'falling through the cracks', and each would be fairly isolated from other planes, hence isolating them from extinction level events such as asteroids or eruptions since an asteroid may only be 1 km in size on the 4d plane. This will allow your sailors to encounter quite a few permeations of ancient animals. Dinosaurs, giant fungi etc. Come to think of it, maybe the Kraken and other sea monsters live in these adjacent 3d slices, but somehow know how to navigate the 4d space to hunt in our world.

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  • $\begingroup$ This answer is underrated. $\endgroup$ Dec 25 '20 at 4:45
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    $\begingroup$ @Mayonnaise2124 Absolutely agree. Ingenious and thoughtful too. $\endgroup$
    – a4android
    Dec 26 '20 at 7:03
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Glitch?

Glitch?

https://www.youtube.com/watch?v=qPrVuS1U4oM

In the amazing lifelike videogames now available, it is sometimes possible to glitch outside the map. There are videos dedicated to finding and exploring these areas which for some reason I find engaging.

Characters can fall out of the world. Often one can see the world fading away above you, or partially rendered objects. What happens next varies - people fall back into the world, or fall away indefinitely, or land at some platform far below, or just fall and fall.

The physics governing falling out of the map is probably the physics governing the rest of the game. Although not necessarily.

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  • $\begingroup$ Thats an oddly creative point of view lol, and I could certainly make use of this $\endgroup$ Dec 24 '20 at 4:38
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    $\begingroup$ @OVERWOOTCH You might want to look into the Simulation Hypothesis. Although it’s primarily a philosophical argument there are some serious physicists that have given thought to experimentally assessing it. $\endgroup$
    – Joe Bloggs
    Dec 24 '20 at 13:22
  • $\begingroup$ @OVERWOOTCH - really sailing off the world is pretty bizarre. But coming at this silly premise from a physics engine game design perspective seems legit for a college essay. That keeps your creative entry then switches to a degree of practicality one might want in a physics major. $\endgroup$
    – Willk
    Dec 24 '20 at 17:28
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Actually I was thinking of asking a similar but less ambitious question.

The planet would be perfectly spheriodal, but there would be an ocean which was higher than the rest of the ocean and had a waterfall leading to the lower ocean.

My theory would be that a vast section of ocean floor between continents was tilted by plate tectonics so that one side was tilted up and formed a long, narrow island connecting two continents, while the other side of the sea floor was lowered to make deeper water.

The upthrust ocean floor and the new long narrow island it made cut off the section of ocean from the other oceans and made it an inclosed inland sea, still very salty.

The water level of the inclosed sea would rise and fall relative to the long narrow island with seasons, weather, and tides. At least at some times, it would rise high enough to flow over the lowest section of the enclosing island and erode it away, making it lower and taking away the sides.

So it might eventually become a giant waterfall with water up to a few feet deep depending on the height of the sea and at least thirty or forty miles wide, so that the shores on each side would be invisible over the horizon from the section in the center of the waterfall.

I suppose that if the water at the very edge was 1 foot deep and 1 foot wide, and 40 miles or 20,800 feet wide, that would total 20,800 cubic feet. And presumably several such volumes of water would go over the falls every second. If ten such volumes fell every second, it would be 208,000 cubic feet per second, or 7,928 cubic meters per second.

Several waterfalls on Earth have greater flow rates, but none of them is so wide that its two shores can't be seen from each other.

So I guess the water flow in such a wide waterfall would have to be ten times as great, 2,800,000 cubic feet or 79,280 cubic meters per second, or maybe a hundred times as great, 28,000,000 cubic weet or 792,800 cubic meters per second, or maybe a thousand times as great, 280,000,000 cubic feet or 7,928,000 cubic meters per second.

And so I suppose the ocean below the waterfall would have to be large enough that evaporation from it would be enough to replenish the inland sea above the waterfall as fast as water flowed over the waterfall. Or mabye the flow of water vapor from the lower ocean might not be as large as the water flow over the falls, if the upper ocean was only high enough to flow over the falls during one season of the year.

So if a ship is trying to reach the other side of the upper ocean, and reaches where the current starts to pull it toward the giant waterfall, the crew may think that is good and it will help them reach the lands on the other side of the ocean faster.

None of them would fear falling off the side of the world, because they would know, correctly, that their world is a sphere. For example seeing approaching objects come over the horizon at sea whould show them that their world was round.

They wouldn't fear falling off until they heard the roar from the giant waterfall and the current became too swift for them to turn back and escape. Then they might curse the evil gods who they thought had temporarily turned a round world flat just to destroy their exploring expedition.

Then the bottom might be riped off their ship on the lip of the waterfall and they would plunge over it.

So I was thinking of asking a question about whether such a situation, with a higher ocean and a lower ocean separated by a long land mass broken by a waterfall too wide to see land on either side would be possible.

Since there are 60 seconds in a minute, 3,600 seconds in an hour, 86,400 seconds in a day, and 31,557,600 seconds in an average Julian calendar year, the flow of the giant waterfall could be 2.5018865 times 10 to the 14th power, or about 250,188,650,000,000 cubic meters per year. Two hundred fifty trillion cubic meters. That would have to be the proportion of water evaporated from the higher and lower ocean which rains on lands along the higher ocean and flows into it each year, if the waterfall is flowing all year.

Some 496,000 cubic km (about 119,000 cubic miles) of water evaporates from the land and ocean surface annually, remaining for about 10 days in the atmosphere before falling as rain or snow.

https://www.britannica.com/science/hydrosphere/The-water-cycle[1]

Since a cubic kilometer contains one billion cubic meters, 496,000 cubic kilometers is 496,000,000,000,000 cubic meters, about twice the possible annual volue over the water fall.

Thus a planet like Earth might possibly evaporate enough water for part of the evaporated water to rain and keep the giant waterfall going part time or all year long.

And I was thinking about asking a question for someone to design a planet with the right geography and hydrology for such a feature.

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  • $\begingroup$ A narrow stretch of land connecting two continents is called an isthmus, not an island. Look to that connecting the two Americas. A minor point. Otherwise a great answer. Plus one. $\endgroup$
    – a4android
    Dec 26 '20 at 6:54
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How about a sudden drop in the earth‘s shape (no matter what shape that would be). This would require the water to suddenly lose height and thus creating a drop that the ship might follow. Imagine a sudden rift, like throwing the content of a cup of water up in the air and seeing it falling down.

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  • $\begingroup$ That actually works, but by what mechanism will the change in shape occur? $\endgroup$ Dec 24 '20 at 4:28
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within neolithic human prehistory, 5500 BCE:

https://en.wikipedia.org/wiki/Black_Sea_deluge_hypothesis

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  • $\begingroup$ Welcome to Worldbuilding SE! Skimming the Wikipedia article, this might be something that could be turned into an answer, but as of now it's not what we would call an answer, just a link. HOW would this translate to "sailing off the edge"? It would be interesting to read, and might earn you some upvotes :) $\endgroup$
    – EdvinW
    Dec 24 '20 at 15:28
  • $\begingroup$ I thought it was obvious. You sail to the end of the Mediterranean ocean, and find an enormous waterfall, two or three kilometres wide, and as many high. If your boat goes over that, you might as well be falling off the edge of the world. And this actually happened, recently enough in human prehistory, that people might actually have had boats, and been sailing around in them. $\endgroup$
    – Noah
    Dec 24 '20 at 17:41
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Let's start by clarifying the concept of "falling". Falling is movement towards the strongest source of gravity.

Now all we have to do is provide a stronger source of gravity than the planet's core.

For example, perhaps the outer boundary of a passing rogue black hole's gravity well skimmed across the surface of the ocean just long enough to grab your ship such that it "fell" up into the sky.

Moments later, as the black hole moved away, the strongest source of gravity might return to being the planet's core, allowing your airborne ship to experience a second kind of falling; this time ocean-ward.

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Hmm, an edge you can sail over. Tricky. Ok, how about this.

  1. The planet is a flat circular disc. A concave/bowl shape might make things easier in some respects, but we'll stick with flat for simplicity.
  2. You ideally want the body of water at the outside to be somewhat narrow, try to minimise the volume of water exerting lateral pressure. If the story allows for a continuous ring of land to isolate that body of water from the main oceans then all the better.
  3. Gravity operates mostly as "normal" ... as normal as you can get on a flat earth at least. This means water is pulled down towards the disc, but the closer you get to the edge the more angled gravity is. The goal here is to try and further reduce the outward water pressure.
  4. Since a flat earth would usually have its satelites orbiting circles above the disc, you might want to exploit that to have a hand full of moon like things spread out to form a tidal pull and, you guessed it, reduce outward water pressure.
  5. The water level needs to be above the disc edge for sailing off to work sensibly, but it would be best to keep it to a minimum, something like a U shaped channel around the edge would make the most sense.
  6. There's no getting around the fact you need some kind of fairly unscientific explanation for why the atmosphere doesn't all fall off the edge, so that will get leveraged as well.

What this is trying to sell is that the pressure applied, to whatever keeps the water and air in, is low enough to be believable while not being strong enough to prevent something solid from escaping by simply building up a bit of momentum.

As for what holds the water in and lets the ships out, I can think of three valid options.

First the least interesting, just have the magic bubble hold the water back with pure hand waving force. It's like having a force field around the world, simple and to the point but also a bit disappointing.

Second is a common excuse from real flat earthers, place a wall of ice around the edge. You'd argue that outside of the magic bubble is extremely cold but not immediately a vacuum. That way, when water tries to flow over the edge it comes in contact with the outside and freezes, producing a relatively thin ice wall that has just enough strength to hold back the water (remember, inside the bubble is warm, so the ice has a steep temperature gradient preventing it from expanding inward). For your character to go over the edge, they just have to ram into that ice wall and break through it, allowing them to go over the edge with a little bit of water while the gap refreezes behind them. This option requires the least hand waving of "what keeps the stuff in" since you only need to magic this heat bubble.

The third, and possibly most interesting solution, is the opposite of the ice wall. The bubble keeps in the air and only that, it also sucks back in any escaping gas. The idea is that as water tries to pour over the edge, it immediately boils after hitting the vacuum, evaporating to water vapour that the bubble can drag back in to become clouds. It would require some careful thinking on how things are arranged to handle the potential build up of water vapour underneath the disc but the aesthetic of the water boiling away to contribute to the water cycle is satisfying.

It's worth noting that any way of leaving a flat disc would come with the issue of being able to breath, since you probably don't want too much of the planets air to be wasted off the edge. The second option would allow for some grace before the air got too thin to breath, but they instead have to worry about not freezing to death.

I did have a fourth idea where the disc is flat but a "double sided" surface arrangement, with a sharp edge where the water would fall "down" the edge and be pulled inside near the middle, fuelling a geyser or aquifer spring system to maintain the water level. Pulling that off would be pretty convoluted though, I'm not sure it's worth the effort unless specifically using part of it as a plot point.

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Even if a ringworld type place, you run into basic problem that if you have an atmosphere it is going to be leaking off at absurdly high rate compared to your ship if edge of the world is at sea level rather than vastly above where little atmosphere.

Blackhole passing by, everything tends to be pulled apart by vast difference in gravity, so rather than ship falling off world it would be ship and those in it stretched and torn apart and entire world likely destroyed as well.

You can do something in a "Truman Show" type world or advanced aliens have a "human" zoo where the world is revealed to be fake, or if you are in a "The Matrix" like virtual world, or a "The Prisoner"/mission impossible mindgame where people are drugged and deluded into thinking they are one place (ocean) when they are really somewheres else (waterfall on wide river).

One TV show did what turned out to be con artist fake "time travel" using drugs and hypnosis to make people imagine/hallucinate that they traveled back in time... similarly a drugged up person might be convinced to hallucinate he is going over the edge of world, or dropped into some portal to another world.

"God's must be crazy" movie has hero think he is throwing the "cursed" bottle over end of earth when he throws it over a waterfall.

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  • $\begingroup$ Not just a waterfall but God's window a real place that thanks to long high cliffs and persistent clouds below the cliffs could easily be mistaken for the end of the world. $\endgroup$
    – John
    Dec 26 '20 at 4:53
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Some answers have touched on real-world events that come close (Black Sea/Mediterranean, Mediterranean/Atlantic) but neither really do what you want, so lets make it a bit more extreme:

There is a gigantic volcanic caldera. At one point the lip of the caldera is slightly above sea level, on a waterway connected to the ocean. (It was once a river, but the caldera cut it, water no longer flows.) The waterway is subjected to tidal bores--and the peak of the bore overtops the lip.

Stray into the river mouth at the wrong time and you can get swept over.

The walls are steep, the volcano is still somewhat active--the water that goes over evaporates, if somehow the crew of the ship survives they can't climb out and the environment is awfully hostile. This would certainly seem like the edge of the world until it had been figured out.

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With a custom water cycle:

enter image description here

A ship could totally fall over the "End of the world", it then plummets several km to the very hot rocks below. All water that falls over is turned to steam, then clouds, then rains back down on the plateau.

This design has an extra cruel feature to drag out death: Ships could briefly get stuck on the "End of the world", as the sea shallows quite quickly so a deep keeled boat could run aground. They're well and truly stuck there though. They can't swim against the current, and on the next cycle of the tides or change of the winds and they're lifted in the rising water and thrown over.

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  • $\begingroup$ unless the flow into the pit is very low, the pit would be cooled down rapidly because you can stuff so much thermal energy into water to turn it into steam. $\endgroup$
    – Trish
    Dec 26 '20 at 12:35
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Without building a new world, tweaking terminology and definitions could be helpful here, it all depends on what you mean by "sea", "ship" and "falling off"

If sea can be considered as the densest part of the atmosphere, atmosphere could also be considered as the gazeous layer of the sea.

Falling is experiencing no acceleration along a geodesic in spacetime, whether it is curved by mass, or not.

A spaceship can reach escape velocity while still being in the upper layers of the gaseous part of the seas. By doing so it climbs Earth's gravity well and falls off it, never coming back.

Astronauts fell off the edge of the world using the Apollo ship.

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