Fosse Dionne Underground Spring is located in the heart of Burgundy, in Tonnerre. Every second, the flow of spring water is 311 liters. When it rains, this number is up to 3,000 liters. The Romans used the water of the stream to live and eat. The Celts considered it a sacred stream, and the French used it as a public bath during the 1700s.
For centuries, people in Burgundy have wondered about the source of the spring. But no matter how carefully they researched, so far they still have no answer as to where Fosse Dionne began. Many explorers even tried to dive to find the source, but were not found, or died. Many people believe that this is the portal to a mysterious world. Others believe that this is the den of a giant snake or that its source is in the nearby Morvan Mountains.
In the 18th century, a stone wall was built around the mouth of the underground stream and there was an opening for the water to flow out. As time went on, the mystery made Fosse Dionne all the more intriguing. In 1974, two professional divers found narrow passages like limestone while diving deep into the stream. But instead of finding the source, they found their own burial ground. 22 years later, the town of Tonnerre hired another diver to see what lurks beneath the surface. In the end, this person also died.
In 2020, the mayor of Tonnerre hires professional diver Pierre-Éric Deseigne to solve the mystery. This person had dived more than 70m underground, swam deep into a 370m rock cavity but then turned back and could not find anything more.
There are 16 trains a day running between Paris and Tonnerre, about 2.5 hours for a distance of more than 160 km. In addition to the stream, Tonnerre has many other attractions. Guests can book a local tour to move, shuttle during the day.