This Mysterious Underground Spring Is Breathtaking, But Its Shadowy Depths Hide A Dark Secret

With its fluctuating colors and constant water flow, the Fosse Dionne spring is one of France’s most stunning attractions. Housed inside an ancient amphitheater, people continue to peer into the well today, curious as to what lies at the bottom. However, this beautiful landmark actually harbors a shocking secret beneath the surface.

Located in Tonnerre, France, the Fosse Dionne is arguably the community’s most famous spot. The spring can be found in the middle of a street, between a number of different houses. But the amphitheater itself is a few feet below the road’s surface, so you can look down into it.

The Fosse Dionne produces a lot of water, which spews out from the spring’s stone wall. In total, as the BBC website has noted, roughly 68 gallons is released per second. Alongside that, The Travel webpage has reported that another 82 gallons flows into the attraction within the same time frame.

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Keeping that in mind, local residents have been asking themselves the same questions for hundreds of years now. Just where is this water coming from? And why is there so much of it? At first glance, the Fosse Dionne has no obvious source, yet some have theorized that the Morvan mountain region could be responsible.

If that’s the case, though, you’d never be able to tell from just looking at the surface of the Fosse Dionne. Indeed, the water’s still and rather peaceful before flowing out at the edge of the amphitheater. But that’s not to say that the conditions remain the same throughout the year.

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To explain more, the Mayor of Tonnerre spoke to the BBC back in December 2019. During that conversation, Dominique Aguilar also touched upon the importance of the Fosse Dionne to her community, citing its fascinating alterations. From the start, she admitted, “We do not know the origin of this spring.”

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Aguilar continued, “This pit indeed is something very important to the city of Tonnerre. It changes every day, according to the weather, the sky, the flow. Its colors vary from blue to green, to brown.” That latter point could serve as an indicator to the source, especially as the alterations are seasonal.

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However, while the Fosse Dionne’s origins remain a mystery, some bizarre myths have circulated about it throughout the years. For instance, certain individuals claimed that if you reached the pool’s floor, you’d find a gateway to different realms. Alongside that, stories of a dwelling serpent creature did the rounds as well.

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Indeed, ladies who went to the Fosse Dionne to clean their clothes long ago believed that holy men had been dispatched to kill the cold-blooded beast. But this particular myth didn’t just spread through spoken words. The claims were seemingly immortalized at one of the local churches, via a unique statue.

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On that note, a French historian named Pierre Millat offered some additional information about the legend. While speaking to the BBC from the church in December 2019, he revealed, “The statue that we have behind us represents two elements [from the Fosse Dionne myth]. Saint-Jean de Réaume and the famous basilisk.”

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Millat continued, “[The creature was] a cross between a lizard and a rooster.” The statue certainly reflected that bizarre combination, with the basilisk sitting on a small platform next to Réaume. It also had a collar around its neck, which suggested that the monk caught it in the Fosse Dionne.

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As it turned out, though, Réaume wasn’t just seen as a standard monk. According to Millat, he was revered as a man who could pull off acts of great wonder. When looking at his background, the historian explained, “He was a seventh century monk, who came to the area in 635 A.D. to clean up the Fosse Dionne.”

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Millat then revealed, “[The Fosse Dionne] was a putrid swamp in Merovingian time. Saint-Jean is positioned in a monastic context. They were pioneers and they were considered capable of performing miracles. And the proof was the Basilisk of the Fosse Dionne.” It’s an interesting story, but people aren’t just drawn to the spring for that reason.

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Alongside the strange myths, the Fosse Dionne has a rich history too, with its importance dating as far back as both Celtic and Roman times. Over the course of this latter period, residents saw the pool as the ideal place to access fresh water. From there, the local communities subsequently treated it as their centerpiece.

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The Celtic people, meanwhile, didn’t just see the Fosse Dionne as a useful resource. In fact, their views lined up with some of the more fantastical tales about the spring, claiming that it was a “sacred” spot. Yet in 1758 the area underwent a major transformation thanks to the Chevalier d’Éon.

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At that stage, the Fosse Dionne turned into a washing area for the residents of Tonnerre. Due to that, the amphitheater was constructed around the edge of the pool, giving the “washerwomen” a base to work from. Incredibly, those roles were still being performed over 150 years later, with the izi.Travel website claiming that they earned an hourly rate of three francs.

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But of course, the amphitheater wasn’t the only addition to the Fosse Dionne during that time. The spring has a wall, too, which was constructed at the same time as the building. And if that wasn’t enough, a simple system was also put in place to manage the levels of water.

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You see, the stone barrier was equipped with a nozzle that people could switch off at any time. As a result of that, they didn’t have to waste any water while cleaning their clothes in the Fosse Dionne. It was essentially a large tap, years before such a tool ever became more widespread.

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As for the washerwomen, they helped to propagate some of the more outlandish myths when they gazed at the Fosse Dionne. Due to its unknown size, those individuals felt the need to discuss what might be down there. Regardless of the tales, though, the mystery surrounding the spring’s origins continues to linger.

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But while there isn’t a concrete answer, one man did offer up an assessment of the Fosse Dionne’s early history. His name was Pierre-Éric Deseigne, and he plied his trade as a diver. During a conversation with the BBC, he speculated that the pool sprung up at the dawn of humankind.

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Deseigne said, “[Fosse Dionne] is a very old spring. [It was] created a long time ago, with a human presence that goes back to the origin of man.” Whatever the case may be, people are still drawn to this incredible landmark in the center of Tonnerre, taking in its natural beauty.

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However, certain individuals have tried to investigate the Fosse Dionne’s source in the last few decades. To get to the bottom of the conundrum, they looked to dive into the spring and explore its dark depths, which can be seen above the surface. One such attempt was recorded back in 1974.

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No other swimmers had tried to explore the pool before that point, as officials noted that it would be incredibly risky. Following the dive in 1974, someone else tried their luck some 20 years later. Then, after all of that, Deseigne was given the green light in the fall of 2018.

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Armed with a camera, Deseigne was able to capture some jaw-dropping images of the Fosse Dionne’s depths. From what he saw, the spring resembled an underwater cave, housing several narrow passageways. A particularly dicey area was found 98 feet below the surface, as the diver navigated a very tight space.

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Keeping that in mind, Dominique Aguilar explained why Deseigne’s voyage was only the third on record. When she spoke to the BBC, she looked back on the previous attempts and unveiled a dark secret. Aguilar said, “A dive took place in 1974, during which an accident occurred and two people died.”

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“The last dive was allowed in 1996, and unfortunately [it was] a fatal dive,” Aguilar continued. “Since this last, fatal accident, dives have been banned. [But] when I arrived in 2014, I decided to allow a dive again. With Pierre-Éric Deseigne, we put a program in place to clean up the Fosse Dionne, change the breadcrumb trail, and do research to create technical drawings.”

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So, that brings us back to Deseigne’s dive in 2018. After navigating the passage at 98 feet, he faced an even tighter spot at 124 feet below the surface. Once he got through that, the diver then reached a sandy floor. But that wasn’t the bottom of the spring. In fact, he went a little deeper before coming back up.

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In total, Deseigne ventured over 980 feet from the Fosse Dionne’s entrance. It was a remarkable effort, yet some 12 months later, he went back to the spring again. When he returned to the surface, he shared his thoughts with the BBC in December 2019, touching upon his role in mapping the area.

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Deseigne said, “It is customary to say that the cavity only exists if it is mapped. So, we take notes on a notebook, we draw plans to make a cut [in] a section of the gallery. We take note of the distances, the depths, the axis and the azimuth of the gallery, which makes it possible to work out where you are.”

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At that point, Deseigne referred back to the tight spaces he encountered when describing the start of the dive. Then, the mapper dropped a bombshell. He continued, “We have the great descent, and that brings us to the famous narrow. This is where the accidents occurred. The people didn’t manage to come out again.”

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Meanwhile, Deseigne shared another eye-opening bit of information during his chat with the BBC. By the end of the dive in 2019, he’d gone down to a depth of just under 260 feet. That area is referred to as the “unknown zone,” but he noted that there was something else ahead.

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Deseigne recalled, “Before really getting into the deep zone, there is a big vertical fault. And when I arrived at the terminus, I could not resist. I still had enough gas. So I let myself go and I said to myself, ‘This time it’s me who falls into the unknown.’”

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“You feel very happy to be [in the unknown zone],” Deseigne admitted to the BBC. “To be the first and think, ‘The light never came here. I’m the first man to see that.’ I noticed that it continues onwards, the gallery is not blocked. It leaves hope for the future.”

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While Deseigne looks ahead, though, there are other springs around the globe that are just as perilous as the Fosse Dionne. For example, Jacob’s Well is one of the most infamous attractions in Texas. The body of water might not seem too bad at first glance, but it’s quite unnerving below the surface.

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Indeed, Jacob’s Well has an opening that spans roughly 30 feet in depth, before swimmers reach the base. At that stage, the spring’s underwater caves are revealed. To give you an idea of how dangerous this area is, the ZME Science website reported that “specialized divers” are the only ones permitted to enter them.

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If that wasn’t clear enough, Jacob’s Well also has a number of warnings dotted across the area, too. They read, “Stop. Prevent your death! Go no further.” Unfortunately, that hasn’t put people off in the past. As a result of that, the Treehugger website claimed that up to nine divers have lost their lives in the spring.

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Although the Fosse Dionne doesn’t have that many fatalities attributed to it, Deseigne’s footage proves that it’s another place divers should avoid. Speaking of the latter, he concluded his interview with the BBC by touching upon the mystery of the pool’s source. Despite making inroads underwater, he was no closer to an answer.

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Deseigne said, “I think in terms of the [Fosse Dionne] mystery, it’s not certain that we will ever get to the end of it. In fact, the springs and the galleries do not offer themselves easily. We have human and technological limits.” On that note, Aguilar held a similar view herself.

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Given how deep Deseigne had gone without finding any clues, Aguilar seemed to admit defeat. In the mayor’s mind, it was a conundrum that someone else might be able to solve in the future. She told the BBC, “Today Pierre-Éric Deseigne has gone 29 feet further [than he did before].”

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Aguilar then concluded, “So, there are still many questions. What is beyond? We do not know. The mystery remains intact and we still don’t know how to explain it.” Whatever the source may be, Tonnerre’s star attraction isn’t going anywhere just yet. Not while the water still flows, at least.

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