Fosse Dionne: The Deadly Cave That Keeps Its Secrets Hidden

Fosse Dionne, a spring in the Burgundy region of France, known not only for its eerie beauty but also for a series of tragic incidents that have occurred over the years. The location is famous for its strange underwater labyrinth and troubling history.

Location and Description of Fosse Dionne

Fosse Dionne, also known as the “Divine Pit,” is located in the town of Tonnerre in France. Unlike many remote springs, this one is surrounded by civilization, making it particularly unique. A narrow stream runs through the town and opens up into a perfectly circular spring.

Surrounding the spring are Renaissance-era homes and castles that look down into the basin. The surface walls of the spring are manmade, constructed in the 18th century, forming a circle around the water. The water in the spring appears a clear blue and green, except for one area where the blue deepens, revealing the entrance to an underground cave system.

The Legends Surrounding Fosse Dionne

The mystery of Fosse Dionne has been enhanced by the legends surrounding it. The first recorded mention of the spring was in a text from 659 AD, describing a monk’s journey to rid the town of a mysterious creature said to lurk beneath the waters. This cryptid, known as the basilisk, was described as a hybrid between a rooster and a lizard.

Later legends continued to build the spring’s supernatural reputation. One such story tells of a young girl who was pursued by an unknown figure, only to be saved by the Virgin Mary. The girl fled into the spring, which was believed to be a portal to the underworld. In later tales, the Devil was even seen entering the spring in the form of a black knight.

The Water Source and Dive Attempts

The Fosse Dionne’s exact water source remains a mystery. While it is known that some of the water comes from rainwater trickling down the surrounding hills, a study in the 20th century confirmed that part of the flow comes from an underground section of the Lane River. Despite efforts by the local government to trace the source, the mystery persists.

The spring has drawn the attention of divers for over a century, with many hired by the town to explore its depths. Early dives were conducted in the 19th century, but the equipment at the time was not advanced enough to explore beyond the immediate basin.

Early Exploration

• 1908: Regular dives were performed, but none ventured beyond the basin.
• 1955: The first dive with tanks was attempted, but only a small distance from the entrance was reached.
• 1962: Significant advancements in diving equipment allowed a team to make it past the narrow entryway. They reached a depth of 92 feet (28 meters), but further progress was limited.

Tragic Losses in Fosse Dionne

In 1963, a team of divers tried to explore further but tragically lost two of their members. The cave’s enormous size became apparent when the Le Guen brothers reached 1,181 feet (360 meters)—ten times farther than previously thought. Despite their success, the cave still appeared endless.

In 1974, two divers hired to explore beyond the Le Guen brothers’ depth disappeared without a trace. Their bodies were never found, and the cave’s dangerous currents and tight spaces likely contributed to their deaths.

• 1996: Another diver tragically lost his life while exploring the cave.
• 2019: Pierre-Eric Deseigne was granted permission to explore the cave, reaching the same point as Jolivet, but had to turn back.

The Dangers of Diving in Fosse Dionne

Fosse Dionne is not a typical cave dive. There are several aspects that make this location particularly dangerous for divers.

Flow Rate and Current

The average flow rate at the spring is 82 gallons per second (311 liters), but during floods, it can increase to 793 gallons per second (3,000 liters). The current becomes concentrated in narrow passages, which requires divers to use substantial strength and energy to move through.

Tight Restrictions and Visibility

The entry to the cave is about 8 feet (2.5 meters) wide but narrows quickly. Divers must squeeze through a 2.5 by 1.3 feet restriction, which is extremely claustrophobic. To pass, divers must remove their air tanks and push them through ahead of them, all while maintaining their regulator and managing poor visibility conditions due to silt disturbance.

Just beyond this tight section, there is another restriction that is even smaller. Most of the divers who lost their lives in Fosse Dionne were found in these tight sections, underscoring the inherent risks.

Depth and Decompression Stops

After passing through the narrow restrictions, divers face a vertical ascent. This requires them to perform decompression stops to avoid decompression sickness. Further into the cave, divers must descend to 98 feet (30 meters) before ascending again. This cycle of depth changes makes it difficult to dive deeper into the cave without multiple air tanks and gas mixtures.

The need for decompression stops and the challenge of navigating tight spaces has made it nearly impossible for anyone to explore the Fosse Dionne further.

Conclusion: The Secrets of Fosse Dionne

The Fosse Dionne remains one of the most dangerous and mysterious dive sites in the world. Despite numerous attempts to explore its depths, it remains largely unexplored. The combination of tight restrictions, strong currents, and unpredictable depths makes it one of the most challenging and fatal cave dives.

The secrets of the Fosse Dionne are likely to remain hidden, as the cave’s dangers continue to deter even the most experienced divers.