They painted magnificent cave paintings. They mastered fire and used tools. And now we know they constructed complex buildings deep within subterranean caves, and they did it more than 175,000 years ago. No, we're not talking about early humans. Neanderthals did all this.

A team of archaeologists led by Jacques Jaubert at the University of Bordeaux in France has just completed an archaeological examination of a mysterious find: the rubble of two ancient Neanderthal-made buildings meticulously crafted from stalagmites. The site is located 1,000 feet into a dark, twisting cave 30 miles outside what is now Toulouse in southwestern France. The discovery is the first of its kind and, the researchers say, radically alters the understanding of Neanderthal culture. Jaubert's team outlines their exploration today in a paper in the journal Nature.

"Because Neanderthals were the only [human-related primate] group present in western Europe at that time, the discovery provides the first directly dated evidence for Neanderthals' construction abilities. It also shows that Neanderthals explored underground," writes Marie Soressi, archaeologist at Leiden University in the Netherlands (not involved in Jaubert's archaeological examination), in an essay accompanying the study.

3D reconstruction of the structures in the Bruniquel Cave. Xavier MUTH - Get in Situ, Archéotransfert, Archéovision -SHS-3D, base photographique Pascal Mora

The scientists found more than 400 stalagmites of similar size meticulously stacked to form the walls of two mysterious circular structures. (Refresher: Stalagmites are the mineral spears that rise up from the ground; stalactites are the hanging ones.) Although Bruniquel Cave was discovered in 1992 and clearly had primitive paintings near the cavern's mouth, only now have scientists like Jaubert been able to delve and explore into the cave's depths. The structures discovered today were found in the dark reaches 1,102 feet from the cave's entrance.

One of the ring structures is about 7 feet in diameter. The other is a bit more egg-shaped and varies between 22 and 15 feet in diameter. The rings have been blackened, cracked, and reddened—evidence that they were heated by fire. Jaubert's team confirmed the chemical fingerprints of fire during an analysis of the rings, and also uncovered what appear to be heated bones found inside.

Just what the Neanderthals were building deep within this French cave is not altogether clear. The rings could have been manically crafted during a single, accidental underground visit. They could have been frequently visited and played a more important function, as a refuge or spiritual destination, in Neanderthal activities. For now, this is just guesswork.

Drilling into the stalagmitic floor inside one of the structures in the Bruniquel Cave. Michel SOULIER - SSAC

Regardless what exactly the rings were used for, Jaubert argues their mere construction is a wonder—a feat far more complex than many anthropologists might believe Neanderthals are capable of.

"This certainly was a collective work, and required at least a minimum of social organization," Jaubert told Popular Mechanics. "This task really was a project, which was likely discussed between several [Neanderthals]. Then it took organized work and the assignment of tasks. Some had to carry torches, some had to move and transport materials, some had place them in this specific configuration, and so on." The discovery paints a picture of Neanderthals as far more socially complex than the classic (although now discredited) Far Side image of a crude, dim-witted species.

"Between 175,000 and 50,000 years ago we know of at least two humanities: one in Africa (Homo sapiens), and one in Eurasia (Homo neanderthalensis)," Jaubert says. "We now know they were both at similar stages in technical development, with regard to tools, knowledge, spirituality, equipment for hunting and fishing." And because today's discovery pre-dates any known human cave exploration, he says, "it is curious to note that it was not Humans (Homo sapiens) who were the first to explore far into caves, or to draw animals."

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