The Fosse Dionne in the town of Tonnerre, France, has kept its mystery for many years due to its undiscovered source. The eerie water that swallowed three divers is believed to be a portal to another world. Being a karstic spring, the Fosse Dionne was used as drinking water by the Roman emperors. The Celts, one of the oldest communities in Europe, believed this place to be a sacred formation.

In the 1700s, the French built an amphitheater around the Fosse Dionne using stones. Throughout history, many people have taken advantage of this water for bathing and washing laundry. However, despite its tranquil appearance, there are many legends circulating around Fosse Dionne. During the Middle Ages, the locals believed there was a snake living in the depths of the Fosse Dionne. Some even thought it was a gateway to another world.

The source plays a significant role in the stories of Saint Jean de Rèome who arrived in the region in the 7th century. The basilisk creature, which struck fear among the people during the Middle Ages, was said to have the power to kill with its gaze. According to legend, Saint Jean de Rèome killed the creature named basilisk while cleaning the swamps at Fosse Dionne, making the source usable again. Despite many eerie rumors surrounding the divine pit known as Fosse Dionne, it has a peaceful appearance.

The biggest mystery of Fosse Dionne is its water source. Like other karstic sources, it is fed from underground limestone cave networks. However, despite the strong water flow, nobody has fully solved where the source originates from. Those who wanted to find out have paid heavy prices in the past. 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 dreadful fate befell another diver in 1996. As a result, diving into Fosse Dionne was banned for a while.

After numerous assessments of the dangers of the source, the mayor of Tonnerre entrusted professional diver Pierre-Éric Deseigne to conduct research. Pierre-Éric Deseigne discovered underwater passages measuring 370 meters in length, documenting his journey with underwater footage and safely returning. However, he could not find out where the source originated from either. Despite all efforts, Fosse Dionne, initially appearing calm, continues to preserve its mystery.