The Wookey Hole catchment drains the southern side of North Hill and the northern side of Pen Hill. At over 9.4 km in length, the longest cave on Mendip is Swildon's Hole, a classic swallet cave system near Priddy. First descended in 1901 it is one of the earliest systematically explored caves in Britain. The streamway initially cascades steeply down a large vadose canyon to Sump 1, a totally water-filled section. Beyond it the passage gradient slackens and the streamway changes to a series of phreatic loops. The troughs are marked by ten further sumps before becoming impassable at a twelfth. Above lie series of abandoned passages at several levels, representing former courses of the stream.
St Cuthbert's Swallet nearby is a similar swallet cave, 6.7 km long, with numerous vadose canyons developed either side of a plunging anticline uniting at depth into a single phreatic streamway.
Extensive collapse and sediment infilling has modified much of the original system, which is well decorated with stalagmites and stalactites. The adjacent Eastwater Cavern (2.5 km long) is a similar, but smaller complex maze. These caves have been traced to Wookey Hole, a 3.6 km long resurgence cave. Part of the cave is open to the public and the cave has been the site of many pioneering cave dives. Here, the River Axe flows through a series of phreatic loops up to 90 m deep within the Carboniferous Limestone before reaching daylight via a series of shallower sumps developed in the overlying Triassic Dolomitic Conglomerate.
Several other small relict caves, notably Hunter's Lodge Inn Swallet, Hunter's Hole and Templeton Pot have been discovered by excavating some of the many sinkholes in the area.