The Yeo rises from a spring near Charterhouse and then disappears under the Limestone of the Mendip Hills before reappearing in Gough's Cave before emerging into Cheddar Gorge. Within Gough's Cave the river forms the largest underground river system in Britain. From a point relatively close to the areas of the cave open to the public, the cave-divers' descent into Sump 1a begins through a tight passage known as Dire Straits. The bottom of that passage opens into the river passage, which is several meters across. This has been explored for downstream, whilst upstream a dive of brings the diver out in a long chamber named Lloyd Hall (which can now also be reached by an alternative, dry, route). Another dive of through Sump 1b, finishing with an ascent through a rising passage, leads to another chamber, long and wide at its widest point, and full of large boulders, called Bishop's Palace. This chamber is the largest chamber currently found in the Cheddar caves. Further on, three sump pools (named the Duck Ponds) lead to Sump 2 which is about deep at its lowest point and long. Air is again reached at Sheppard's Crook, which is followed by Sump 3. This sump is deep and at its bottommost point is about below sea level. Following Sump 3, a wide ascending passage continues for before reaching an impassable blockage, still below the water's surface.