Sully Island

Sully Island is a small tidal island and site of scientific interest in Sully, 450 metres off the northern coast of the Bristol Channel midway between the towns of Penarth and Barry and 7 miles south of Cardiff, Wales, UK. Access to the island is on foot at low tide. It is 14.5 acres in extent. Sully Island is one of 43 (unbridged) tidal islands which can be reached on foot from the mainland of England, Wales or Scotland.

During the 13th century, the island was the base for "Alfredo De Marisco", a Norman pirate known locally as "The Night Hawk". In the Middle Ages the island was well known for its involvement in the local smuggling trade. 

It may be called Sully after the Norman baronial family of Sir Reginald de Sully. 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 rocky causeway connecting the island to the mainland is uncovered for approximately 3 hours either side of low tide, the island being cut off from the mainland for the rest of the day. This makes Sully Island a potentially dangerous place, and many people have been swept to their deaths while trying to leave the island as the tide rises very rapidly. All visitors to the island must exercise care and due diligence. Although now there is a timer near the island which gives visitors tidal times, letting them know if it's safe to cross.

There is evidence that the island was frequently visited by both Romans and Vikings and the remains of a Saxon multivallate promontory fort occupying the eastern end of the Island, on the summit of which is a bronze age barrow. It has been suggested by some that this was an armed stronghold, but it was more likely to have been a defended residence and farm homestead.

In 1899 a hoard of Roman coins, gold and silver was unearthed on  Sully Island.