“Legend has it there is a drowned land beneath the sea in Cardigan Bay. Known as Cantre’r Gwaelod [The Sunken Hundred], in the earliest version of the legend, in The Black Book of Carmarthen, the land is known as Maes Gwyddno.
The land of Cantre’r Gwaelod was said to be very fertile, due to the fact that it was low lying land which was protected from the sea at high tide by a dyke with sets of sluice gates.
The watchman, Seithenyn, got a little (a lot) drunk and fell asleep, forgetting to shut the gates.
The gates were left open, and the sea rushed in flooding the land of Cantre’r Gwaelod. The king Gwyddno Garanhir and some of his court are said to have fled to safety along Sarn Cynfelyn.
Nautical chart by Lewis Morris (1701-1765) of Aberystwyth Bae, originally from Ynys Môn.
It’s said that on still days you can still hear the bells of Cantre’r Gwaelod ringing beneath the waves.”
(For a discussion and analysis of haunted bells and vanishing lands as recurrent motifs in Celtic mythology and storytelling, see chapters 2 and 6 of my first book, Ancestral Recall: The Celtic Revival and Japanese Modernism.)