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 rich ditch systems. Regularly maintained field ditches are often species-rich and diverse. Notable species include Lesser Water-plantain ("Baldellia ranunculoides"), Tubular Water-dropwort ("Oenanthe fistulosa") and Hairlike Pondweed ("Potamogeton trichoides"). The channels and banksides support a rich fauna; rare species include the water beetle ("Hydrophilus piceus") and the soldier fly ("Odontomyia ornata"). Large populations of dragonflies and damselflies occur, including the Hairy Dragonfly ("Brachytron pratense") and the Variable Damselfly ("Coenagrion pulchellum"). 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