This is what we found on Wikipedia regarding the Lyonesse:

Lyonesse (/liːɒˈnɛs/ lee-uh-NESS) is a kingdom which, according to legend, consisted of a long strand of land stretching from Land's End at the southwestern tip of Cornwall, England, to what is now the Isles of Scilly in the Celtic Sea portion of the Atlantic Ocean. It was considered lost after being swallowed by the ocean in a single night. The people of Lyonesse were said to live in fair towns, with over 140 churches, and work in fertile, low-lying plains. Lyonesse's most significant attraction was a castle-like cathedral that was presumably built on top of what is now the Seven Stones Reef between Land's End and the Isles of Scilly, some 18 miles (29 km) west of Land's End and 8 miles (13 km) north-east of the Isles of Scilly. It is sometimes spelled Lionesse.

Lyonesse is mentioned in Arthurian legend, specifically in the tragic love-and-loss story of Tristan and Iseult. It was the home of the hero Tristan (one of the Knights of the Round Table), whose father Meliodas was king of Lyonesse. After the death of Meliodas, Tristan became the heir of Lyonesse, but he was never to take up his inheritance because the land sank beneath the sea while he was away at his uncle King Mark's court in Cornwall. In later traditions, Lyonesse is said to have sunk beneath the waves in a single night, but stories differ as to whether this catastrophic event occurred on 11 November 1099, or 10 years earlier. According to one legend, the people of Lyonesse had committed a crime so terrible that God took his revenge against them and their kingdom. The exact nature of the crime is never specified, but the legend tells of a horrific storm that occurred over the course of a single night, resulting in an enormous wave that swallowed the kingdom.