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
River Wensum SSSI River Wensum SSSI The River Wensum SSSI is a 'whole river' Site of Special Scientific Interest located on the River Wensum that flows through the English county of Norfolk. The river was designated a SSSI in 1993 and a Special Area of Conservation SAC in 2000. It covers of the Upper Wensum from its source close to South Raynham downstream to Hellesdon Mill an area of . The Wensum's upper reaches are fed by springs that rise from chalk and was selected as being one of the best examples of a lowland chalk and oolite river type III. There are