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
Ffynnon Beuno and Cae Gwyn Caves and the bones of extinct pre-iceage animals such as mammoth and woolly rhino. In 1963 it was designated as an SSSI for the palaeontological interest described above and for its species interest as a winter roost of lesser horseshoe bat. Ffynnon Beuno and Cae Gwyn Caves Ffynnon Beuno and Cae Gwyn Caves are two Scheduled Ancient Monuments which are also designated a Site of Special Scientific Interest (SSSI) in Denbighshire, Wales. Two caves, with entrances close to each other high on a valley side above the Vale of Clwyd. They were first excavated in 1883-5. Human tools have been found