Deep within a French cave where no light penetrates are two curious structures: large rings of stalagmites, some broken and arranged like the rails of old-fashioned wooden fences. When discovered in the early 1990s, scientists didn’t know what to make of the formations, which appeared to be fire-scorched in places. Now, they may have an answer: The rings were built by Neandertals, who learned to explore caves extensively and engaged in complex building behaviors like arranging stones more than 175,000 years ago, much earlier than thought.

The ancient structures—more than 330 meters inside the current entrance to the cave—include a scattering of small, deliberately arranged heaps of stone along with two large rings, one about 2.2 meters across and the other nearly three times that size. The rings and piles are made of about 400 stalagmites of similar size, weighing a total of 2.2 tons. Most of the forearm-sized fragments are roughly cylindrical and were intentionally broken to the proper length, says Sophie Verheyden, a geologist at the Royal Belgian Institute of Natural Sciences in Brussels. That much is clear, she says, because the pieces are missing both their tips and their bases. “I’m a caver, and those structures are something like I’ve never seen before.”

But who made the mysterious rings? Clearly humans, says Marie Soressi, an archaeologist at Leiden University in the Netherlands, who was not involved in the new study. The deliberate arrangement of stones, as well as the size of the rings, indicates that “they clearly weren’t made by bears” wallowing a hollow to sleep in, she notes.

Verheyden and her colleagues used radioactive-dating techniques to analyze the pieces of broken stalagmite, as well as layers of minerals naturally deposited on the structures after they were constructed. The results suggest the structures were built between 175,000 and 177,000 years ago, the researchers report online today in Nature. Previous studies suggest the climate in the region during this time was relatively warm and wet, so the moisture needed to seep through the overlying rocks to create the stalagmites would have been abundant, Verheyden says. And because modern humans didn’t leave Africa until approximately 100,000 years ago, the structures must have been made by Neandertals, she adds.

Team members haven’t yet discovered the Neandertal’s workshop, because most of the missing stalagmite bases and tips aren’t anywhere near the structures. But the researchers did find one hole in the cave floor where a stalagmite had been wrenched up and carried away.

The next big mystery is what the Neandertals used the structures for. In dozens of spots along the rings or on small heaps of stone, the rocks seem to have been discolored by the heat of ancient fires. A thumb-sized fragment of burned bone hints that the ancient cavers cooked meals there.

The find is the first directly dated evidence of Neandertal construction and the first evidence that these humans explored caves deeply, Soressi says. The rings also hint that Neandertals might have also been building structures above ground at the time, though any direct evidence of such structures—likely made of wood, bone, or animal skins—are likely long lost to the elements. The cave structures survived, Soressi suggests, because they were built of long-lasting materials in a protected environment.