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
Deadman SSSI, Somerset surface horizon. It is at these points that bog communities develop, characterised by the floating leaves of bog pondweed ("Potamogeton polygonifolia"), bog bean ("Menyanthes trifoliata") and marsh St John's-wort ("Hypericum elodes"). In places this wet heath has undergone a transition to birch ("Betula" sp.) and willow ("Salix" sp.) carr. This provides an important habitat for breeding nightingales and grasshopper warblers. Deadman SSSI, Somerset Deadman () is a 28.8 hectare (71.2 acre) biological Site of Special Scientific Interest on the edge of the Blackdown Hills, near Buckland St Mary south of Taunton in Somerset, notified in 1987. Deadman is one of