The Enigmatic Fosse Dionne Spring in France Continues to Guard Its Secrets

Main Points

There may be a giant, deadly serpent lurking at the bottom of the Fosse Dionne spring in the Burgundy region of France. Alternatively, the serpent could just be a myth, dreamed up by the ancient people of Tonnerre, where the spring resides. However, no one has ever truly uncovered the truth, as no one has ever been able to explore the depths of the spring.

Fosse Dionne is a karst spring (a karst refers to a region of irregular limestone with sinkholes, underground streams, and caverns, as defined by Merriam-Webster), discharging an impressive 82 gallons (311 liters) of water every second. This is a remarkably high rate for such a spring, though the speed at which the water emerges can vary with the seasons. If you were to visit Fosse Dionne, which means "divine pit," you’d see a circular stone pool from the 18th century, brimming with jewel-toned waters — turquoise, amber, and cerulean — colored by the minerals found in the limestone caves from which the spring's waters flow. One of these caves' entrances is visible at the basin's edge.

Humans have relied on Fosse Dionne for centuries, using its waters for drinking, washing, cooking, and bathing, which has led to a wealth of stories and legends. In the Middle Ages, many believed a serpent resided deep within the spring’s core, while others thought the spring might be a gateway to another realm. The spring is also significant in the lore of St. Jean de Rèome, a 7th-century monk who arrived in 645 C.E. to cleanse the spring, which was then a swamp. According to reports, the monk pulled a basilisk, a creature part rooster, part lizard, from the spring and killed it, making the spring usable again for the local people.

Today, Fosse Dionne looks quite refined from the outside, with its stone basin surrounded by a lavoir, a communal washing area built in the 18th century to shield washerwomen from the elements as they did their laundry in the spring. Yet, beneath the surface, the spring remains as untamed and wild as it was when St. Jean de Rèome came to tame it.

The enduring mystery of Fosse Dionne is the source of its water. A vast amount of water flows from it, and like other karst springs, it emerges from an extensive network of underground limestone caves. Despite numerous attempts, no diver has ever discovered its origin, and several who have tried have never returned.

The first serious attempt to explore the depths of Fosse Dionne occurred in 1974, when two divers ventured into its maze of chambers and narrow passageways. Neither of them returned to share their discoveries. In 1996, another diver made an attempt but tragically lost his life in the spring as well. For many years, diving into the spring was forbidden until 2019, when diver Pierre-Éric Deseigne explored 1,214 feet (370 meters) of passageways. Fortunately, he returned alive, but he did not uncover the source of the spring — nor did he encounter another dimension or a gigantic serpent.

There's still hope.