How would Atlantis work on a Planetary Scale?
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. 710
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. 710