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
Bourne SSSI, Avon Bourne SSSI, Avon Bourne SSSI, Avon () is an 8.47 hectare geological Site of Special Scientific Interest near the village of Burrington, North Somerset, notified in 1992. This site is of considerable importance because it has provided detailed information upon the composition of a north Mendip Pleistocene alluvial fan. An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. At Bourne sections have shown highly weathered gravels overlain by sandy silts and clay loams, the highest levels in the sequence showing evidence