Lost lans are continents, islands or other regions believed by some to have existed during prehistory, but to have since disappeared as a result of catastrophic geological phenomena or slowly rising sea levels since the end of the last Ice Age. Lost lands, where they ever existed, are generally thought to have subsided into the sea, leaving behind only a few traces or legends by which they may be known. The term can also be extended to mythological lands generally, to underground civilizations, or even to whole planets.