[Moltke]

a coastal civilization, which was drowned as glaciers melted after the end of the Younger Dryas period.

I have to ask - they just stood there watching the water rise from their feet until they drowned? If so, good riddance to the fools!

[Salamander]

That's what happened with Hy Brasil.

[Renfield]

I rather imagined that it was only the buildings and other fixed structures that drowned; the people would have fled inland (probably repeatedly every generation or two).