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
Moorlinch SSSI Moorlinch SSSI Moorlinch () is a 226.0 hectare (558.4 acre) biological Site of Special Scientific Interest at Moorlinch in Somerset, notified in 1985. Moorlinch is part of the extensive grazing marsh grasslands and ditch systems of the Somerset Levels and Moors. Lying in the Parrett Basin at the foot of the Polden Hills, the area drains by gravity into the King’s Sedgemoor Drain. The water table is high for most of the year with frequent winter flooding from high ground and surface water remaining on many fields throughout the winter and early spring. Moorlinch contains a good proportion of botanically