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
Writhlington SSSI Writhlington SSSI Writhlington SSSI () is a 0.5 hectare geological Site of Special Scientific Interest near the town of Radstock, Bath and North East Somerset, notified in 1992. This is the site of old mine workings on the Somerset coalfield, including 3,000 tons of Upper Carboniferous spoil from which more than 1,400 insect fossil specimens have been recovered, including the world's earliest known Damselfly. It is a Geological Conservation Review Site, because it has yielded the largest ever collection of Carboniferous insects in Britain. The commonest forms belong to the order Blattodea (cockroaches) and include the extinct families Archimylacris and