Swildon's Hole Cavern
Cave · Bristol
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Swildon's Hole is the longest cave on the Mendip Hills, running over 9,000 metres through crawlways, chambers and active streamways. Part of the Priddy Caves Site of Special Scientific Interest, it has been explored since 1901 and still challenges cave divers, with eleven sumps navigated and a twelfth still impassable. The entrance sits in a small stone blockhouse near Manor Farm.
From Wikipedia
Swildon's Hole is an extensive cave in Priddy, Somerset. At 9,144 metres (30,000 ft) in length, it is the longest cave on the Mendip Hills. It has been found to be connected to Priddy Green Sink and forms part of the Priddy Caves Site of Special Scientific Interest (SSSI). The upper series of the cave compresses many features into a relatively short space. The cave goes far beyond this, however, and the lower reaches of the cave continue to provide challenges for even the most experienced of cave divers. The name may be a corruption of Swithun, an Anglo-Saxon bishop of Winchester, as the land in the area was owned by St Swithin's Priory in Winchester.