Caves of the Mendip Hills traverse from one cave to another through the underground rivers led to the development of cave diving; the first cave dive attempt in Britain took place at Swildon's Hole in 1934. The first successful cave dive in Britain was achieved the following year at Wookey Hole Caves, where the last sump is currently the deepest in Britain at . The cave complexes at St. Dunstan's Well Catchment, Thrupe Lane Swallet, Lamb Leer and Priddy Caves have been identified as geological Site of Special Scientific Interest. There are also caves within the Cheddar Complex SSSI. A selection of the better known
Site of Special Scientific Interest Access to the Countryside Act 1949, but the current legal framework for SSSIs is provided in England and Wales by the Wildlife and Countryside Act 1981, amended in 1985 and further substantially amended in 2000 (by the Countryside and Rights of Way Act 2000), in Scotland by the Nature Conservation (Scotland) Act 2004 and in Northern Ireland by the Nature Conservation and Amenity Lands (Northern Ireland) Order 1985. SSSIs are also covered under the Water Resources Act 1991 and related legislation. An SSSI may be made on any area of land which is considered to be of special interest by