The Bass Rock, or simply the Bass is an island made of steep-sided volcanic rock. It reaches a height of 107 metres (351 feet) at its highest point. The rock lies in the outer part of the Firth of Forth, approximately 2 kilometres (1.2 miles) offshore, and 5 kilometres (3.1 miles) north-east of North Berwick. The rock is currently uninhabited, but historically has been settled by an early Christian hermit, and later was the site of an important castle, which was used as a prison after the Commonwealth period. The Bass Rock Lighthouse was constructed on the rock in 1902, and the remains of an ancient chapel survive. The island is home to a large colony of gannets.