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
Portishead Pier to Black Nore SSSI Portishead Pier to Black Nore SSSI Portishead Pier to Black Nore SSSI () is a 71.8 hectare geological Site of Special Scientific Interest near the town of Portishead in North Somerset, notified in 1952. The Portishead Pier Section is made up of alluvial sandstones which represents the best exposure of Upper Carboniferous rocks in the Avonmouth Coalfield. The cliff and foreshore exposures around Portishead Point provide important exposures of geological structures form during the Variscan mountain building episode (orogeny) in the Carboniferous Period of geological history. Also included are areas which show important exposures of the Devonian sequence and yields