Saint Michael's Mount, pyramid-shaped rocky islet, 21 acres (8.5 hectares), Cornwall, SW England, in Mounts Bay; it rises to more than 200 ft (61 m). A natural causeway connects it at low tide with the mainland. Its Christian history began in 495 when fishermen claimed to see a vision of St. Michael appearing on its summit. A priory built in the 11th cent. and a castle are there.