For about 176,000 years, two large rings of broken stalagmites have adorned the floor of Bruniquel Cave in southwest France. Researchers aren't sure what they were built for, but they do know that Neanderthals were the ones who built them, making them the most complex Neanderthal—and the earliest hominin—structures ever found, Nature reports. 

The cave constructions were first spotted in 1990 by local spelunkers who blasted through the entrance of a cave that had been obstructed for thousands of years. A French archaeologist came across the stalagmite circles 1000 feet from the cave's opening, but he died before he had time to study them in depth. Animal remains dating back at least 50,000 years were found nearby, so it was assumed that the rock formations were around the same age. It wasn't until 2013 that scientists revisited the site and discovered they were even older.

Uranium dating tests put the calcite stone at 176,000 years old. That's remarkable because the first modern humans are believed to have arrived in Europe as recently as 45,000 years ago, and as The Atlantic's Ed Yong points out, "Outside Bruniquel Cave, the earliest, unambiguous human constructions are just 20,000 years old." The builders appear to have snapped some 400 stalagmites from the ground and arranged them into two rings—a smaller one more than 6 feet wide and one as large as 22 feet across. The stones had been stacked into fence-like structures 2 feet high in some spots. The constructions are too advanced to have been made by an animal, which led scientists to conclude they were built by our relatives

It's surprising to find evidence of Neanderthal presence so deep in a cave, where there would have been no natural light. To further add to the mystery, scientists aren't sure what compelled them to build the rings in the first place. The stalagmites and parts of the cave walls show evidence of burning, suggesting the rings were used to contain fires. Whether the fires were built for warmth, protection, or ritualistic purposes still remains unclear. But the discovery does support the recent shift in thinking towards viewing Neanderthals as intelligent as early humans.

[h/t Nature]

All images courtesy of Nature