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
Nagshead SSSI Nagshead SSSI Nagshead () is a biological Site of Special Scientific Interest and is located near Parkend, in the Forest of Dean, Gloucestershire, and was notified in 1972. It lies within the Forest of Dean Forest Park and is part held as a reserve by the Royal Society for the Protection of Birds (RSPB). The SSSI is a relatively small area of the much larger nature reserve of RSPB Nagshead, which is a site owned and managed by the RSPB and the Forestry Commission. The SSSI lies to the east of the Nagshead Plantation and consists of two units of