St Cuthbert's Swallet is the second longest, and most complex, cave on the Mendip Hills, in Somerset, England. It forms a major part of the Priddy Caves system and water entering this swallet re-emerges at Wookey Hole. St Cuthbert's Swallet is part of, and lies underneath, the Priddy Pools Site of Special Scientific Interest. In the citation this is given as St Cuthbert's Cave, which should not be confused with St Cuthbert's Cave in Northumberland. It is named because of its location in the St Cuthbert Out parish of Wells, and was originally called St Cuthbert's Pot.
St Cuthbert's Swallet is a classic example of a deep phreatic cave system at a depth of not less than 85 metres (280 ft) beneath the contemporary water table. It contains inclined bedding plane mazes at many levels, fault-guided rifts and some significant chambers. Nine successive phases of sediment deposition, stalagmite deposition and stream erosion have been recognised at the site, providing an exceptional record of environmental change through the warm and cold phases of the Ice Age.
St. Cuthbert's Swallet is attractive to cavers, for its complexity and size, reaching over 6.7 kilometres (4.2 mi) and a depth of 145 metres (476 ft), and having numerous large decorated chambers, forming phreatic mazes on seven distinct levels. Among its displays are large and impressive calcite groups such as the 'Curtains', 'Cascade', Gour Hall with its 6.1-metre (20 ft) gour, 'The Beehive', Canyon Series and the 'Balcony' formations in September Chamber, perhaps some of the best in Britain. There are also mini-formations including floating calcite crystals, over twenty nests of "cave pearls", and delicate fern-like crystals less than 4 millimetres (0.16 in) long.
The cave has also been identified as an important site for the study of cave insects. Oligaphorura (formerly Archaphorura) schoetti (a type of springtail) is a troglophile, up to 1.7 millimetres (0.07 in) in length and common in many caves. Most specimens are female but a male specimen was collected from St. Cuthbert's Swallet in 1969.