The 25th chamber represents the furthest upstream air surface in Wookey Hole Cave. From here the River Axe rises up from a deep sump where progressive depth records for cave diving in the British Isles have been set: firstly by Farr () in 1977, then Rob Parker () in 1985, and finally by John Volanthen and Rick Stanton () in 2004. The pair returned again in 2005 to explore the sump to a depth of , setting a new British Isles depth record for cave diving. This record was broken in 2008 by Polish explorer Artur Kozłowski, then later again by Michal Marek, on dives in Pollatoomary in Ireland.