In the Mabinogeon, the waters between Britain and Ireland are known as the Sinking Lands, an apparent (very ancient) memory of former dry land, now recalled only by the broken-off submerged forests.
Reminds me of this image:
Stone statue at Insishmor, Aran Islands.
The Seven Stone Rocks are held to be the remains of a city that local fishermen call The Town, while in Mounts Bay in Penzance the remains of a sunken forest can be seen at low tide. Lending weight to the legends is the fact that St. Michaels Mount, in Mounts Bay itself, has the Cornish name of Carrack Looz en Cooz - literally the grey rock in the wood. Cornishmen around Penzance still believe strongly in a sunken forest in Mount's Bay, and archaeological evidence of the forest is visible at very low tides, where petrified tree stumps become visible.