These weren’t natural formations, and they weren’t the work of bears. They were built by people.

Recognizing the site’s value, 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 discovery suggested that Neanderthals were more sophisticated than anyone had given them credit for. 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?

Rouzaud would never know. In April 1999, while guiding colleagues through a different cave, he suffered a fatal heart attack. With his death, work on the Bruniquel Cave ceased, and its incredible contents were neglected. They’ve only now re-entered the limelight because Sophie Verheyden went on holiday.

A life-long caver, Verheyden works at the Royal Belgian Institute of Natural Sciences, where she specializes in stalagmites. She treats them as time capsules, using the chemicals within them to reconstruct the climate of past millennia. So 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?”

She knew that Rouzaud’s date of 47,600 years was impressive but suspect. Carbon-dating is only accurate for samples younger than 50,000 years, so the Bruniquel material was hitting the technique’s limits. They could well have been much older. To get a better estimate, Verheyden assembled a team including archaeologist Jacques Jaubert and fellow stalagmite expert Dominique Genty. 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.”

After drilling into the stalagmites and pulling out cylinders of rock, the team could see an obvious transition between two layers. On one side were old minerals that were part of the original stalagmites; on the other were newer layers that had been laid down after the fragments were broken off by the cave’s former users. By measuring uranium levels on either side of the divide, the team could accurately tell when each stalagmite had been snapped off for construction.

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 are 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.