Cantre'r Gwaelod (also Cantref y Gwaelod, Welsh for "The Lowland Hundred") is a legendary sunken kingdom from Welsh mythology, sometimes called the "Welsh Atlantis." It is said to have been a fertile, low-lying land in what is now Cardigan Bay, west of Wales, stretching between Ramsey Island (Ynys Dewi) and Bardsey Island (Ynys Enlli).
Cantre'r Gwaelod means "the lowland cantref." A cantref, or hundred, was a medieval division of land.
In the sea, about seven miles west of Aberystwyth in Cardiganshire, is a collection of loose stones, termed Caer Wyddno, "the fort or palace of Gwyddno;" and adjoining it are vestiges of one of the more southern causeways or embankments of Catrev Gwaelod.
The story developed between the late 15th and 17th centuries, drawing from older oral traditions and possibly influenced by the Welsh legend of Cantre'r Gwaelod (a sunken kingdom).