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
Foulness SSSI sand-silt flats, saltmarsh, beaches, grazing marshes, rough grass and scrubland". The flats are of international importance for nine species of wildfowl and waders, such as dark-bellied brent geese. Rare plants include soft hornwort and spiral tasselwood, and the site is also important for invertebrates, with 71 nationally rare species. Foulness SSSI Foulness SSSI is a 10,702 hectare biological Site of Special Scientific Interest covering the shoreline between Southend-on-Sea and the Crouch estuary in Essex. It is a key site in "A Nature Conservation Review", and is part of the Essex Estuaries Special Area of Conservation. It covers two Ramsar wetland