Lyonesse is one of the most dramatic legends connected with Cornwall. It is usually described as a lost land that once lay between Land’s End and the Isles of Scilly, before being swallowed by the sea in a great flood. Later accounts describe it as a prosperous kingdom with villages, fields, roads, and 140 churches. Although there is no proof that such a kingdom existed, the story has remained powerful because it brings together folklore, coastal change, local identity, and the idea of a vanished world beneath the waves. The legend became closely aligned with Arthurian literature. In Sir Thomas Malory’s Le Morte d’Arthur, Lyonesse is named as the home of Sir Tristram, the Cornish knight whose story is tied to love, loyalty, and tragedy. Alfred Lord Tennyson later gave Lyonesse an even stronger place in the Arthurian imagination through Idylls of the King, where it becomes the setting for Arthur’s final battle with Mordred. Older Cornish accounts also helped define the legend. William Camden’s Britannia, published in 1586, recorded stories of a drowned land near Cornwall, while Richard Carew’s Survey of Cornwall, published in 1602, referred to Lethowsow, meaning the submerged lands. Carew also linked the story to the Trevelyan family, whose arms are associated with a horse escaping the flood. These early references suggest that stories of lost land and drowned places were already part of Cornish tradition before the Victorian revival made Lyonesse more widely known. The story also has a connection with real changes in the landscape. Submerged forests have been recorded around the Cornish coast, including ancient tree remains exposed at very low tides in Mount’s Bay. Geological studies also show that the Isles of Scilly were once part of a larger landmass, later reshaped by rising sea levels after the last Ice Age. This does not prove the existence of Lyonesse as a kingdom, but it does show why stories of drowned land could have taken root in coastal communities that lived with visible evidence of environmental change. Victorian writers such as Thomas Hardy and Walter Besant helped keep the legend's fire burning, while later artists, poets, musicians, and storytellers have continued to reinterpret it. Today, Lyonesse can be read as folklore, literature, environmental tragedy, and Cornish cultural identity. Whether Lyonesse is a myth, history, or a combination of both, it is one of Cornwall’s most compelling stories of a world lost to the sea.