Of course, some of the ancient Welsh kingdoms are now lost beneath the waves:

- Cantre'r Gwaelod (the Lowland Hundred) in Cardigan Bay, once the kingdom of King Gwynddno Garanhir of Cardigan. Susan Cooper's Silver On The Tree makes great use of the "Lost Land" and King Gwynddno in his glass castle at Caer Wydr.

- Llys Helig, the Palace of Helig ap Glannowg in Conwy Bay. Apparently Helig's lands were drowned as judgement for the crimes of his ancestors.