La Fosse Dionne

One of the highlights of our recent trip was a visit to the Fosse Dionne in Tonnerre, Burgundy. I'd heard about this place but we had never previously visited. It wasn't on our itinerary, but when I realised how close we were going to be I requested a diversion. Luckily we also happened to have plenty of time.

The Fosse Dionne is a karstic spring which emerges right in the middle of the small town of Tonnerre, in the departement (county) of Yonne. It is fed by rain falling on the nearby Jurassic limestone plateau, and is connected to an underground river. The spring is remarkable for the quantity of water flowing through it (an average of 300 litres per second) and the extent of hydrological network, which reaches a distance of at least 40 kilometres.

The presence of this spring is the reason the town was established here. In the 18th century a public laundry (lavoir) was created around it.

The name is a corruption of the Old French phrase for 'divine spring'. It is thought likely that the name of the divinity associated with the spring in Celtic times was a goddess called Divona, who is associated with the watery underworld. 

During floods the flow of water can be ten times the average, but in summer it can reduce to about a third.

In 1758 Louis d'Eon had the lavoir constructed around the spring. There is a 14 metre diameter pool surrounded by a semi-circular roof to protect the laundry women. A low wall separates the spring itself from the trough used for laundry, in order to prevent pollution. Nearby households were engaged to supply the ash needed in the laundering process.

The spring consists of a vast cistern with an opening onto a drowned gallery, which you can see from the surface. There have been a number of exploratory dives conducted in the 20th century, but access is now rarely granted because of how dangerous the site is, with very narrow passages. Two divers have died and the extent of the exploration is 400 metres. The most recent dive was in 2019, when a new guide line was laid down. The diver reached nearly 80 metres in depth and just a bit further than any previous exploration.