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
Ringdown SSSI, Somerset found here. Ringdown SSSI, Somerset Ringdown () is a 4.0 hectare (9.9 acre) biological Site of Special Scientific Interest west of Burnworthy in the Blackdown Hills of Somerset, notified in 1990. The site is located on a valley side in the Blackdown Hills where outcrops of both Cretaceous Upper Greensand and underlying Triassic Keuper Marls occur. Amongst the vegetation present are two species which are very restricted in South West Britain, white beaksedge ("Rhynchospora alba") and dioecious sedge ("Carex dioica"). This site is the only known location for the latter in Somerset. A colony of the small pearl-bordered fritillary ("Boloria