The Sinking Lands, which lay to the west of Britain (between Britain and Ireland) are mentioned in the Mabinogeon; the stumps of apparently suddenly submerged forests have been found off Wales. Also, most are familiar with Lyonnesse. Winnie Churchill's history of, hmm, Britain mentions the submerged forests and whatnot and that the North Sea floor was once dry land. (':
And much smaller events of this kind have taken place historically, here and there:
Clare Places: Islands: Mutton Island or Enniskerry (9th century catastrophe in Ireland)
Clare County Library | prior to November 19, 2005 | staff writer
Posted on 11/18/2005 2:58:58 PM EST by SunkenCiv
http://www.freerepublic.com/focus/f-chat/1524751/posts
[snip]
According to the "Annals of the Four Masters" the island was once called Fitha Island and it formed part of the mainland until the day "the sea swelled so high that it burst its boundaries, overflowing a large tract of country, and drowning over 1,000 persons." This happened on March 16th, 804. Some reports describe it as an earthquake, others as a tidal wave when "the sea divided the island of Fitha into three parts." These three islands are Mutton Island, Inismattle (or Illanwattle) and Roanshee (or Carrig na Ron). There is a fourth island in the area called Carraig Aolacan.
[unsnip]
"The Sinking Lands, which lay to the west of Britain (between Britain and Ireland) are mentioned in the Mabinogeon; the stumps of apparently suddenly submerged forests have been found off Wales. Also, most are familiar with Lyonnesse."
When Beleriand was broken and sundered by the sea...