Ancient Islands of Lyonesse

The Ancient Islands of Lyonesse, known also as "The Elder Ises",  are a part of the Commonwealth of the Principality and one of the great legends and myths of this world of ours. The legend of a sunken kingdom appears in both Cornish and Breton mythology. In Christian times it came to be viewed as a sort of Cornish Sodom and Gomorrah, an example of divine wrath provoked by unvirtuous living, although the parallels were limited in that Lyonesse remained in Cornish thought very much a mystical and mythical land, comparable to the role of Tir na nÓg in Irish mythology.[citation needed]
There is a Breton parallel in the tale of the Cité d'Ys, similarly drowned as a result of its debauchery with a single virtuous survivor escaping on a horse, in this case King Gradlon. The Welsh equivalent to Lyonesse and Ker Ys is Cantre'r Gwaelod, a legendary drowned kingdom in Cardigan Bay.
It is often suggested that the tale of Lyonesse represents an extraordinary survival of folk memory of the flooding of the Isles of Scilly and Mount's Bay near Penzance.[3] For example, the Cornish name of St Michael's Mount is Karrek Loos y'n Koos - literally, "the grey rock in the wood". Cornish people around Penzance still get occasional glimpses at extreme low water of a sunken forest in Mount's Bay, where petrified tree stumps become visible. The importance of the maintenance of this memory can be seen in that it came to be associated with the legendary British hero Arthur, although the date of its inundation is actually c.2500 BC.