In Welsh folklore , the Lowland Hundred (or Cantre'r Gwaelod in Welsh) was a tract of fertile land stretching northwards from Ramsey Island to Bardsey Island over what is now Cardigan Bay to the west of Wales. Its capital was Caer Wyddno, seat of the ruler Gwyddno Garanhir.

It was defended from the sea by a ditch called Sarn Badrig (St. Patrick's Street), over which a Keeper of the Embankment held charge. One of these keepers, called Seithenyn, was a notorious drunkard and carouser, and it was through his negligence that the sea swept through the open floodgates, ruining the land.

The ruins of a city were supposedly visible as late as 1770, when Wiliam Owen Pughe reported seeing sunken human habitations about four miles off the Welsh coast, between the rivers Ystwyth and Teifi .

Some decades later, Rev. James Yates presented a paper called A Notice of a Submarine Forest in Cardigan Bay, reporting the existence of submerged tree stumps "along the coast of Merionethshire and Cardiganshire, being divided into two parts by the estuary of the Dyfi , which separates those two counties. It is bounded on the land side by a sandy beach and a wall of shingles ... Among the trees of which this forest consisted is the Pinus sylvestris, or Scotch Fir ; and it is shown that this ancient tree once abounded in several Northern counties of England."

The church bells of Cantre'r Gwaelod are supposed to ring out in times of danger, according to a folk tradition.