happened with a bang:
A prehistoric "Atlantis" in the North Sea may have been abandoned after being hit by a 5m tsunami 8,200 years ago. The wave was generated by a catastrophic subsea landslide off the coast of Norway. [The Storegga Subsidence, to geologists.]
Analysis suggests the tsunami over-ran Doggerland, a low-lying landmass that has since vanished beneath the waves. "It was abandoned by Mesolithic tribes about 8,000 years ago, which is when the Storegga slide happened," said Dr Jon Hill from Imperial College London. “The impact... would have been massive - comparable to the Japanese tsunami of 2011.” The wave could have wiped out the last people to occupy this island.Which strikes me as a great way to get stories of lost undersea kingdoms started.