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I have a world, and when the story begins, I want it to almost completely covered by water. There are islands scattered across the planet reasonably frequently, and most of the oceans are very shallow. Only one of the islands has a small society of human-like aliens living on it.

What I need is a way that the planet got that way. At some point in the past, I want there to have been larger islands, more on the scale of continents, that a much larger and technologically advanced society lived on.

So here are the things I need:

  1. How could a planet get from having large continent-sized landmasses to only small islands with only one island inhabited? My current thought process is on the natural-disaster route, but I'm open to other creative solutions like the original society doing it for a reason (if that's your answer, provide a reason).
  2. Could an isolated population on an island survive for a long period of time if isolated like this through a natural disaster or whatever you come up with?

The best answer will address both questions, preferably with reference to a similar historical event or scientific evidence that such a thing could happen. I understand that this actual event hasn't happened in our experience, so if you don't have evidence that's fine.

Some things that might help:

  • I haven't yet decided if the original society was able to travel in space. Feel free to use that in your answer (say, they know of impending disaster and leave the planet, but some people are left behind on accident/purpose).
  • I'm very open to creative solutions that might incorporate something I haven't thought of. If you have a way to get from Point A (technologically advanced society) to Point B (small community living on an island, alone on the planet), I want to hear it.
  • The minimum size of the original landmasses is constrained by "couldn't walk across in a month" (and these people have an island mentality, so they can't even conceive of something even that large) so I'm hoping for something at least the size of Australia, but ask if you need the original landmasses to be smaller and I'll see what I can do.
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  • $\begingroup$ Just a note, it may be good to constrain each topic to one question, to keep answers concise as well. $\endgroup$ – Enthus3d Nov 30 '20 at 12:47
  • $\begingroup$ Thanks, @Enthus3d, I'll try to do that in the future. $\endgroup$ – Benjamin Hollon Dec 1 '20 at 0:12
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Ice age and global warming

During the ice ages water is trapped at the poles due to freezing. This is an incredible amount, freeing up a lot of land mass. In warmer periods this ice melts, flooding it again. This has happened several times. You could argue that some of it was still flooded during an ice age, as layers of ice would reach to New York from the North pole if I recall correctly, but the ocean is retreating.

Although these work for Earth, it'll obviously need to be adapted for your planet. Increasing water or have the landmass decreased (by creasing it more for example) will offer less landmass, but still plenty for life to thrive on. Increasing and decreasing the temperature difference of the ice age and warm age, possibly over a longer time, will give you first enough land mass to thrive on, which can then be reduced to small islands due to the floods. I suppose in extremer scenario's you might not even need to change the amount of landmass too much of our current worldview to flood most of it. It just needs to be layed out a big more optimally.

I think lots of civilizations have been separated from each other over time. Either by disasters or simply by arriving there by accident. You don't get a civilization on Easter Island easily in earlier time periods. Or the thousands of islands of Indonesia. Some had boat connections, but I'm pretty sure many tiny civilizations have been cut off and survived for long times.

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  • $\begingroup$ While this does work, I'm looking for something more permanent. Ice ages are definitely very cyclical and it would probably be very easy for the original civilization to see it coming long in advance and prepare for it. +1 for the historical examples of isolated communities, though, those are helpful $\endgroup$ – Benjamin Hollon Nov 29 '20 at 10:06
  • $\begingroup$ @Benjamin can be more or less permanent. Like the Earth, the ice age warm period cycle could heat up relatively over time, making the ice ages not cold enough to free the land. $\endgroup$ – Trioxidane Nov 29 '20 at 12:28
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Let's start with current day Earth for easy reference. A smallish world with lots of water but still plenty of dry land. Add a civilization of about our current level, with extremely limited space flight but a promising future.

Now fall back through time about a million years to a moment when an enormous river of space borne ice crystals began its slow fall into our sun's gravity well. Watch its long journey until just a few years from now, when its sunward course finally intersects our little planet's orbital path. Watch as hundreds of millions of cubic miles of water vaporize against our outer atmosphere, before settling down upon the surface as a planet wide rain storm that lasts for hundreds of years.

By the time that the space river passes by our orbit, the ocean levels have risen so high that only the peaks of the highest mountains still escape the waves. On the highest of these mountain-top islands, the last remnants of a water-logged humanity struggle for survival. Their technology is long gone, but they still have some level of maritime skill and their new world is rich with fish.

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  • $\begingroup$ Definitely a good solution. Do we have any scientific evidence that such "space rivers" exist? If so, this definitely sounds good enough for me. Also, if it's lasting "hundreds of years" what kind of problems might that cause for the civilization that's caught in the rainstorm? (though now that I think about it, this might be a good thing to cause them to flee the planet like I mentioned as a possibility) $\endgroup$ – Benjamin Hollon Nov 29 '20 at 7:08
  • $\begingroup$ Ice crystals can exist in space. Here is a link. And nebulae clouds can contain water ice. link. which could be drawn into the tail of an enormous comet or rogue planet and thus travel interstellar distance. But as for it being concentrated enough to flood a planet...? I have no proof that that is possible. Sorry. $\endgroup$ – Henry Taylor Nov 29 '20 at 7:25
  • $\begingroup$ No problem, thanks for your help! I'll wait a couple days to make sure there are no better answers people come up with, but I think your solution is definitely viable for my story! $\endgroup$ – Benjamin Hollon Nov 29 '20 at 8:09
  • $\begingroup$ How come Earth's crust was not melted by the enourmous energy liberated by "hundreds of millions of cubic miles of water" falling down from outer space? (That's the general problem with water being delivered from space; water is heavy, and big objects falling down from space tend to heat things up spectacularly.) $\endgroup$ – AlexP Nov 29 '20 at 10:14
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    $\begingroup$ Yeah, lengthening the time-frame though would cause an issue for my story since I was thinking that even a span of hundreds of years was a stretch. It's definitely a good answer, but I'll probably end up using the other, just because it's not actually a process I describe, just one people are trying to deduce from the evidence, and an ice age melting is probably a more plausible explanation to people from Earth since they've seen it happen on their planet $\endgroup$ – Benjamin Hollon Nov 30 '20 at 0:12

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