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
North Somerset Levels have been designated as Sites of Special Scientific Interest. Tickenham, Nailsea and Kenn Moors SSSI is an extensive SSSI in the northern part of the Levels. Yanal Bog, a calcicolous mire near Sandford is designated for its plant communities, within which grow Black Bog-rush ("Schoenus nigricans") and Blunt-flowered Rush ("Juncus subnodulosus"), two plant species rare in southwest England. A number of plant species which are otherwise scarce or absent in the Bristol region are found in high concentrations on the North Somerset Levels, including Water Horsetail ("Equisetum fluviatile"), Rigid ("Ceratophyllum demersum") and Soft ("C. submersum") hornworts, Thread-leaved ("Ranunculus trichophyllus"), Common