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 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. The site has long been renowned for its botanical interest and records date back to the County Flora of 1893. The woodland supports a particularly high diversity of vascular plants, including populations of the nationally rare plant Purple Gromwell "(Lithospermum purpurocaeruleum)" and the scarce Angular Solomon’s seal "(Polygonatum odoratum)". Large areas of King’s Wood were replanted during the 1960s with beech Fagus sylvatica and a