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
West Moor SSSI diversity of the ditches. West Moor SSSI West Moor () is a 213.0 hectare (526.3 acre) biological Site of Special Scientific Interest on the River Parrett in Somerset, notified in 1985. West Moor lies south of Curry Rivel, Langport, and Drayton, and northwest of Kingsbury Episcopi and is part of the extensive grazing marsh grasslands and ditch systems of the Somerset Levels and moors. The site contains some of the most diverse aquatic plant communities in the country. Rhynes often have a rich aquatic flora Frogbit ("Hydrocharis morsus-ranae"). A rich invertebrate fauna, with many nationally and locally rare species is