One of the most interesting sites of this area, this pit is the second most important hydrogeological water-gap in France. Its ecosystem is unique and its flora very rich.

This water, flowing continuously at 140 meters above sea level, has not yet revealed the mystery of its source. In 1758, it has been converted in a wash house.

Discover the legends of the Fosse Dionne

This spring of Vaucluse, with its changing though perpetual flow, roused the curiosity of our ancesters, who considered it as "divine" ("fons divona" evoluted subsequently into the word Dionne). It was said traditionally that the Fosse Dionne was bottomless. According to some legends, it gave access to the nether regions, or it was the living place of a snake with a deadly stare (the basilic), which was living there until a bishop, Saint-Jean de Réôme, succeeded in chasing it away.

It is probably there that the first inhabitants settled, around this perennial flowing water, among dry limestone plateaux. In the Gallo-Roman period, the Fosse Dionne provided water to the inhabitants of the oppidum of Tornodurum, situated on an overhanging high relief - this is testified by the remains of several flights of steps from this period.

In 1578, the Chevalier Louis d'Eon converted the spring into a wash house, with a semicircular roof and a double enclosure, which gave it its present appearance, and made it much more accessible. The Fosse Dionne soon became the headquarters of the washerwomen of Tonnerre. With their beetle, kneeling on a wooden box lined with straw, they worked from noon to midnight, hanging their sheets on the beams for a salary which, in 1920, did not exceed 3 francs per hour. In 1908, during the month of May, were put additional beams as draining racks, as well as small chimneys in the wall to "stew" the laundry. The imps put their ear on the flues to listen to the washwomen's colourful gossips.

This spring forms a basin in the bottom of which we can see a 2,5 m high drift which, at a depth of 28 m, becomes a narrow channel. The drift goes on along 360 m, reaching a depth of 61 m. This French spring is one of the most difficult ones to explore because of its narrowings, its clay whirlpools, its depth and the power of its flow.

Situated at 140 m above sea level, the Fosse Dionne has been stirring up curiosity for a very long time (several divers have perished in its waters). There is much speculation about the source of this water, particularly abundant in winter. With an annual average flow of 242 liters per second, the spring can have fierce swellings.

Originating from an impluvium of 43 km2 at the south of the town, it stays in the numerous faults of the limestone, which explains its continuous flow. A part of the water comes from the loss of the Laigne (near Chatillon-sur-Seine), through an underground course, still unknown, covering 40 kilometers as the crow flies. Thus, the Fosse Dionne is the second most important hydrogeological water-gap in France - the first being the Fontaine de Vaucluse - as it was proved by colour tests.