Fosse Dionne, Natural spring in Tonnerre, France.
Fosse Dionne is a natural spring in Tonnerre that releases water through a circular stone basin, creating a constant flow of turquoise water. The water emerges from underground into the round opening and disappears back into it.
The spring served as a water source for a Roman settlement starting in the 7th century and was later expanded with an 18th-century washing house. These structures show how the site functioned as an important resource across generations.
The name comes from the Celtic word "Divona," meaning divine, and the spring features prominently in local stories about serpents. These connections to the past shape how people perceive and visit the place today.
The site is freely accessible every day throughout the year without any entry fee. Visitors should know there are no wheelchair facilities and no safety barriers are placed around the water.
Multiple diving expeditions have attempted to find the source of the spring by descending to depths beyond 60 meters, but without success. These failed attempts have preserved the mystery surrounding where the water originates.
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