Sully Island is a small tidal island a kilometre off the coast of Sully, South Wales, between Penarth and Barry. Its name may mean "south lea" or "south pasture" or, like the nearby village of Sully it may be a family name. The rate of tidal rise and fall in the area is the second highest in the world: only that of Bay of Fundy, Nova Scotia is greater. A causeway connecting the island to the mainland is uncovered for approximately 3 hours either side of low tide; the island is cut off from the mainland for the rest of the day. This makes Sully Island a dangerous place and many people have been swept to their deaths while trying to leave the island as the tide rises.

Other facts

• A Saxon fort lies on the island's south-eastern headland. Some people believe that this was a Saxon stronghold, while others believe it was a defended homestead rather than a full settlement;

• The Barracks is the name of a rocky beach between Barry and Sully, Vale of Glamorgan, in South Wales. The beach became an important archaeological site after the fossilized remains of a dinosaur footprint dating back 200 million years was found several decades ago. Bendricks Rock has the only known Upper Triassic dinosaur – Tetrasauropus – footprint site in Britain; in addition, the beach displays a remarkable collection of fossils found amongst its rocks.

• The waters around Sully Island are fished for species such as cod, whiting, pouting, dogfish and bass;

• During the 13th century, the island was the base for "De Marisco", a Norman pirate known as "The night hawk". In later years, it was known for its involvement in the local smuggling trade.