Fosse Dionne in the town of Tonnerre, France, has kept its mystery for many years due to its undiscovered source. The eerie water, swallowing three divers, is believed to be a portal to another world. Fosse Dionne, a karst source, was used as drinking water by Roman Emperors. The Celts, one of Europe’s oldest communities, believed it to be a sacred formation.
In the 1700s, the French constructed an amphitheater around Fosse Dionne using stones. Throughout history, many people used this water for bathing and washing clothes. However, despite its tranquil appearance, there are many legends surrounding Fosse Dionne. During the Middle Ages, townsfolk believed a serpent lived in the depths of Fosse Dionne. Some even thought it was a gateway to another world.
In the 7th century, stories of Saint Jean de Rèome play a significant role in the legend of the source.
The Basilisk creature, which terrified people during the Middle Ages, had the power to kill with its gaze. Usually depicted as half rooster, half lizard, this creature was said to possess this deadly power. According to legend, Saint Jean de Rèome killed the creature named Basilisk in Fosse Dionne while cleaning the marshes, making the source usable again. Despite many terrifying rumors, Fosse Dionne, meaning divine pit, has a serene appearance.
The biggest mystery of Fosse Dionne is the source of its water. Like other karst sources, it is fed from the network of underground limestone caves. However, despite the strong flow of water, no one has fully unraveled where the source comes from. Those who tried to discover this in the past paid a hefty price. The first attempts to explore Fosse Dionne date back to the 1970s. In 1974, two divers trying to navigate through narrow and winding tunnels never returned. The same horrifying fate occurred in 1996 to another diver. Therefore, diving into Fosse Dionne was banned for a period.
Following evaluations, the Mayor of Tonnerre appointed professional diver Pierre-Éric Deseigne to conduct research. Pierre-Éric Deseigne, who discovered the 370-meter underwater passages, documented his journey with underwater footage and returned safely. However, he too could not find out the source of the water.
Despite appearing calm at first glance, Fosse Dionne continues to preserve its mystery despite all risky attempts to uncover it.