La Fosse Dionne

The Fosse Dionne is a karst spring in Tonnerre, in the Yonne, developed as a wash-house. In 1758, Louis d'Éon, father of the Chevalier d'Éon, had it turned into a basin about 14 metres in diameter, protected by a half-rotunda roof. The site has been a listed historic monument since 1920. Its name perhaps comes from the Celtic goddess Divona or from the Latin fons divina, the divine spring.

Its average flow is about 300 litres per second, with strong variations: from 87 litres in August to 619 litres in January. It drains an underground network of more than 40 kilometres, fed by rain seeping through the limestone plateaux and by river losses such as the Laignes. The gallery first plunges at 45° to 32 metres' depth, then explorations reached nearly 80 metres, through narrow and dangerous passages. The basin can be visited freely in the heart of the old town.

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