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
South Exmoor SSSI South Exmoor SSSI South Exmoor () is a 3132.7 hectare (7742.3 acre) biological Site of Special Scientific Interest in Devon and Somerset, England, notified in 1992. The site consists of several fragments of land on the south side of Exmoor. In Devon the site includes Barcombe Down and North Molton Ridge, southeast of South Radworthy, Molland Common, north of Molland, and West Ansty and East Ansty Commons. In Somerset the site includes Brightworthy Barrows and part of Withypool Common, Winsford Hill and Haddon Hill, all within the Exmoor National Park. The site contains extensive areas of heathland including lowland communities