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The Fosse Dionne is a famous and beautiful spring opening in the centre of Tonnerre, a little town 197 km south-east of Paris. It does not look much like a cave from the postmark itself, but on the spot the underground flooded passage is quite visible. This legendary spring is the outlet of the supposed second longest subterranean water drainage in France with 43.5 km (after Fontaine de Vaucluse).

The cave was explored in 1981 down to its present end by Eric Le Guen of the Spéléo-Club de Paris (length: 360m; depth: 61m). The cave is considered as one of the most difficult and dangerous to explore one can imagine, because of the accumulation of different factors: length, muddy waters and hence no visibility, very narrow passages, cold water, strong current, depth and profile with successive ups and downs. In 1962 two divers died in the very first part of the cave.

A colloquium on cave diving and the speleological sciences was organized at Tonnerre by the Spéléo-Club de Paris on the 30th and 31st of October 1982.