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 Norfolk Coast Site of Special Scientific Interest Cley and Salthouse Marshes. The NWT also manages the Holme Dunes NNR. In addition to Blakeney Point, the National Trust owns land at Brancaster Staithe. The SSSI covers 7,700 ha (19,027 acres) and is additionally protected through Natura 2000, Special Protection Area (SPA) and Ramsar listings, and is part of the Norfolk Coast Area of Outstanding Natural Beauty (AONB). Scolt Head Island and the coast from the Holkham NNR to Salthouse are a Biosphere Reserve. The SSSI is designated as a Special Protection Area for birds for its variety of coastal habitats. The large breeding colonies of Sandwich terns and