Ah ha! I have posted a good several times about Cantref (or Cantre'r) Gwaelod. But I've never managed to get any traction on here, when I have.
Cantref Gwaelod supposedly disappeared sometime around the 6th century AD. It was the kingdom of
Gwyddno Garanhir, kept behind a giant floodgate which held the sea back. A 'Cantref' was a medieval term for a land division. It closest translates as 'Hundredth' but could also be seen as a County Kingdom, such as others from this period in history.
The earliest reference known of it comes from
Llyfr Du Caerfyrddin - The Black Book of Camarthen. However, that 13th century book (written entirely in Welsh) is both incomplete (pages literally having fallen out and been lost over the centuries) and also being a collection of poetry and folklore. It is not a factual document. There the kingdom is referred to by another name -
Maes Gwyddno - which best translates as the plain of Gwyddno. Named for Gwyddno Garanhir.
In that telling of the story it is a 'well-maiden' named
Mererid , who neglects her duties and allows the well to overflow, causing the kingdom to flood from within.
The version of the story of Cantref Gwaelod from roughly the 17th century onwards tells a different story. The kingdom is described as a low-lying land fortified against the sea by a dyke,
Sarn Badrig, with a series of sluice gates - closed for high tide, and used to drain water from the kingdom at low tide.
The kingdom's Capital and seat of power was
Caer Wyddno, a settlement speculated to have been located somewhere north-west of modern-day Aberystwyth.
In this version of the tale the blame is laid at the feet of
Seithenyn, who is one of two individuals responsible for the maintaining and operating of the gate. One night, while it's his turn, Seithenyn gets blind drunk and forgets his duty. The kingdom is flooded.
In some tellings of this version Seithenyn is a Prince of the Realm - though not Gwyddno's Son. In others he is merely a contemporary of King Gwyddno.
Seithenyn is also listed in the
Triads of the Island of Britain as one of the "
Three Immortal Drunkards of the Isle of Britain".
If Cantref Gwaelod *
did* exist it would have formed a land mass over much of what is modern day Cardigan Bay, linking seamlessly with the modern mainland, with Gwyddno also being king over land which sits in modern day Ceredigion. In fact in 18th century texts he is referred to as 'King of Ceredigion'.