Cantre'r Gwaelod (also spelled Cantref Gwaelod or Cantref y Gwaelod, meaning "The Lowland Hundred" in Welsh) is a legendary sunken kingdom from Welsh mythology, often called the "Welsh Atlantis." It is said to have been a fertile, low-lying land in what is now Cardigan Bay (west Wales), stretching roughly between Ramsey Island (Ynys Dewi) and Bardsey Island (Ynys Enlli), with its capital at Caer Wyddno.
Cantre'r Gwaelod literally translates as "the lowland cantref" (a medieval administrative unit meaning "hundred").
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).