In February 1990, Bruniquel Cave was discovered when a 15-year-old boy named Bruno Kowalsczewski, footsteps echoed through the chambers of the Cave for the first time in tens of thousands of years. The cave is located in France's scenic Aveyron Valley, but its entrance had long been sealed by an ancient rockslide.

The story of the cave's discovery:

Kowalsczewski's father had detected faint wisps of air emerging from the scree, and the boy spent three years clearing away the rubble. He eventually dug out a tight, thirty-meter-long passage that the thinnest members of the local caving club could squeeze through. They found themselves in a large, roomy corridor. There were animal bones and signs of bear activity, but nothing recent. The floor was pockmarked with pools of water. The walls were punctuated by stalactites (the ones that hang down) and stalagmites (the ones that stick up).

The cave seemed valuable and the caver brought in archaeologist Francois Rouzaud. Using carbon-dating, Rouzaud estimated that a burnt bear bone found within the chamber was 47,600 years old, which meant that the stalagmite rings were older than any known cave painting. It also meant that they couldn't have been the work of Homo sapiens. Their builders must have been the only early humans in the south of France at the time: Neanderthals.

The Neanderthals were sophisticated

 The discovery suggested that Neanderthals were more sophisticated than anyone else. They wielded fire, ventured deep underground, and shaped the subterranean rock into complex constructions. Perhaps they even carried out rituals; after all, there was no evidence that anyone actually lived in the cave, so what else were the rings and mounds for?  

After Rouzaud's death in April 1999, the work on the Bruniquel Cave ceased, and its incredible contents were neglected.

Sophie Verheyden went on holiday.

"When I announced the age to Jacques, he asked me to repeat it because it was so incredible."

A life-long caver, Verheyden works at the Royal Belgian Institute of Natural Sciences, where she specializes in stalagmites. When she learned about Bruniquel Cave, while visiting the region on holiday and seeing a display at a nearby castle, she had only one thought: Why hadn't anyone dated the broken stalagmites themselves?"

In 2013, they got permission to study the site and crawled into it themselves. "I'm not very big, and I had to put one arm before me and one behind to get through," says Verheyden. "It's kind of magical, even without the structures."

Their date? 176,500 years ago, give or take a few millennia.

"When I announced the age to Jacques, he asked me to repeat it because it was so incredible," says Verheyden. Outside Bruniquel Cave, the earliest, unambiguous human constructions ar just 20,000 years old. Most of these are ruins-collapsed collections of mammoth bones and deer antlers. By comparison, the Bruniquel stalagmite rings are well-preserved and far more ancient.

 "When you see such a structure so far into the cave, you think of something cultural or religious."

Why did they build the rings and mounds? The structures weren't foundations for huts; the chamber contains no stone tools, human bones, or any other sign of permanent occupation, and besides, why build shelter inside a cave? "A plausible explanation is that this was a meeting place for some type of ritual social behavior," says Paola Villa from the University of Colorado Museum.

"When you see such a structure so far into the cave, you think of something cultural or religious, but that's not proven," adds Verheyden. Indeed, despite some fanciful speculations about cave bear cults, no one really knows.

Nor is it clear how the Neanderthals made the structures. Verheyden says it couldn't have been one lone artisan, toiling away in the dark. Most likely, there was a team, and a technically skilled one at that. They broke rocks deliberately, and arranged them precisely. They used fire, too. More than 120 fragments have red and black streaks that aren't found elsewhere in the chamber or the cave beyond. They were the result of deliberately applied heat, at intensities strong enough to occasionally crack the rock. "The Neanderthal group responsible for these constructions had a level of social organization that was more complex than previously thought," the team writes.

Neanderthals made tools, used fire, made art, buried their dead, and perhaps even had language. "The new findings have ushered a transformation of the Neanderthal from a knuckle-dragging savage rightfully defeated in an evolutionary contest, to a distant cousin that holds clues to our identity," wrote Lydia Pyne in Nautilus.

Bruniquel Cave 

We know that 400,000 years ago, some ancient hominins chucked their dead into a cave at Sima de los Huesos, but there's no evidence of the careful constructions in Bruniquel. There's evidence of painting and sculpture within caves, but none older than 42,000 years. There are signs that Neanderthals used caves, but nothing to suggest that they frequently ventured deeper than sunlight.

"I think we have several lines of evidence showing that the cognitive abilities and behaviors of Neanderthals were complex," says Marie Soressi from Leiden University. "But we had no direct evidence of their ability to build. That changes the picture for me. It's puzzling to find such structures so deep inside the cave."

To solve these puzzles, Verheyden wants to start cutting into the cave's floor. It has been covered by layers of calcite, which may conceal specimens that hint at the chamber's purpose. Verheyden also notes that the entrance they've been using cannot possibly have been the only one. "We're crawling through this small thing and there are bear hollows in the cave. I don't think the bears went in that way!" she says. "There must have been some other passage that collapsed."