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
Site of Special Scientific Interest (but no other sites) are subsequently notified as geological SSSIs, except some that coincide with designated biological SSSI management units. A GCR site may contain features from several different topic blocks, for example a site may contain strata containing vertebrate fossils, insect fossils and plant fossils and it may also be of importance for stratigraphy. Geological sites fall into two types, having different conservation priorities: exposure sites, and deposit sites. Exposure sites are where quarries, disused railway cuttings, cliffs or outcrops give access to extensive geological features, such as particular rock layers. If the exposure becomes obscured, the feature could