Jaskinia Wielka Śnieżna (Tatra Mts)
Poland's longest and deepest cave system (about 23.8 km, 824 m vertical range) ends in sumps far below the entrances. Syfon Dziadka at −784.8 m is the lowest point reachable without diving; beyond the historical bottom, Syfon Dominiki, divers passed the Dominiki and Beaty sumps between 1972 and 1985 into air-filled passages. The deepest explored point of the whole cave is the Jeziorko X sump, dived to −7.1 m, which puts it at −808 m. Dye tracing showed the water emerges at the Lodowe Źródło spring. This is expedition-scale cave diving with days underground, not a day trip.
depth: 0–7 m