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 virtue of its fauna, flora, geological or physiographical / geomorphological features. SSSI notification can cover any "land" within the area of the relevant nature conservation body, including dry land, land covered by freshwater. The extent to which an SSSI/ASSI may extend seawards differs between countries. In Scotland an SSSI may include the intertidal land down to mean low water spring or to the extent of the local planning authority area, thus only limited areas of estuaries and coastal waters beyond MLWS may be included. In England, Natural England may notify an SSSI over estuarial waters and further adjacent waters in