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
Deacon Hill SSSI Deacon Hill SSSI Deacon Hill SSSI is a 35.4 hectare biological Site of Special Scientific Interest in Pegsdon in Bedfordshire. It is in the Chilterns Area of Outstanding Natural Beauty, and it is part of the Pegsdon Hills and Hoo Bit nature reserve, managed by Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire. The site is calcareous grassland which is rich in plant species, some of which are uncommon. Birds include lapwings and buzzards, and there are butterflies such as dingy and grizzled skippers. There are also the remains of ancient strip lynchet fields. The SSSI covers part of Deacon Hill