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
Tickenham, Nailsea and Kenn Moors SSSI Tickenham, Nailsea and Kenn Moors SSSI Tickenham, Nailsea and Kenn Moors SSSI () is a 129.4 hectare biological Site of Special Scientific Interest between Tickenham, Nailsea and Kenn on the North Somerset Levels, notified in 1995. The soils in the area include both clays of the Allerton and Wentloog Series and peat soils of the Sedgemoor and Godney Series, which are drained by a network of large rhynes and smaller field ditches, which support exceptionally rich plant and invertebrate fauna communities. Exceptional populations of Coleoptera occur with at least 12 nationally scarce species and two nationally rare species, including Britain’s