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
King's Wood and Urchin Wood SSSI variety of conifer species including Douglas fir "(Pseudotsuga menziesii)", Lawson’s cypress "(Chamaecyparis lawsoniana)" and Norway spruce "(Picea abies)" . These crop trees have, however, largely been unsuccessful and the replanted areas are now being overtaken by hardwoods. King’s Wood supports nationally important populations of the rare and endangered greater horseshoe bat "(Rhinolophus ferrumequinum)" and dormice "(Muscardinus avellanarius)", and a nationally scarce Chrysomelid beetle "(Clytra quadripunctata)". King's Wood and Urchin Wood SSSI King's Wood and Urchin Wood SSSI () is a 128.1 hectare biological Site of Special Scientific Interest near the villages of Cleeve and Congresbury, North Somerset, notified in 1990.