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Origin and history

The Dionne pit is a vauclusian karst source located in the heart of Tonnerre, Yonne. Powered by precipitation infiltration into the Jurassic limestone plateau and losses from the Laigne River, it has an average flow rate of 311 litres per second, up to 3,000 litres in flood. Its hydrogeological network, partially explored by divers, extends over 40 km and includes deep siphons and dangerous narrowness.

The source played a founding role in the history of Tonnerre, feeding from the Gallo-Roman era the oppidum of Tornodurum before becoming the nucleus around which the medieval and then modern city developed. His name, Dionne, could come from Divona, the Gaulish goddess of sources and abysses, or from the deformation of Fons Divina, his ancient appellation.

In the 18th century, Louis d'Eon, the father of the famous knight d'Eon, had the spring built into an elegant public wash, with a circular basin of 14 meters in diameter and a half-roof roof. This wash, designed to protect lavenders from weather and avoid water pollution, was listed as a Historic Monument in 1920. Fireplaces were used to produce ash used for laundry.

The speleological exploration of the Dionne pit, initiated in 1955, revealed a complex network with drowned galleries reaching -79.5 meters depth. Despite several fatal accidents (1962, 1996), divers such as Pierre-Éric Deseigne continued his study in 2018-2019, partially mapping the site and installing an Ariane thread. Access remains strictly restricted.

The Dionne pit illustrates both the importance of water resources in historical urban planning and the technical challenges posed by karst networks. Its wash, always visible, bears witness to the domestic and social practices of the eighteenth and nineteenth centuries in Burgundy.

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