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.