A bizarre 41-foot-wide (12.5-meter) circular structure made entirely of wooly mammoth bones was recently unearthed in Russia. Scientists believe that the structure is 25,000 years old. Whether it served as a dwelling, a ritualistic hotspot, or some other purpose is yet unclear.

As the name plainly suggests, hunter-gatherer societies obtained food by hunting, fishing, scavenging, and gathering wild plants and other edibles. It’s believed that before the advent of agriculture, our ancestors lived a nomadic lifestyle, moving in groups of a few dozens of people, consisting of several family units.

But this doesn’t mean that they mindlessly wandered the world. When food was plentiful in an area, it was common for hunter-gathers to stay put in the same place, employing techniques to store food and defending their territory against rival groups.

As out of the ordinary as it may sound, circular structures made from mammoth bones were quite common during the ice age in Eastern Europe.

Recently, Russian paleontologists have discovered the largest one yet: a huge structure made of hundreds of wooly mammoth bones, belonging to as many as 60 different mammoths.

The structure was found at an archaeological site, known as Kostenki 11, which is located by the Don River, close to the Russian city of Voronezh.

Radiocarbon dating suggests that the site is around 25,000 years old, making it one of the oldest mammoth bone structures in history.

Such structures are quite common around Russian and Eastern Europe. In fact, scientists have been discovering mammoth bone structures — albeit of much smaller dimensions — at Kostenski 11 since the 1950s. They’re all circular and flanked by a series of large pits, which may have been used to store food or dump waste.

About 70 such structures are known to exist in Ukraine and the west Russian Plain.

This most recent structure — and the largest found thus far — was first discovered in 2013. For three years the researchers had been excavating the site, employing flotation — a technique that involves water and sieves in order to separate ancient remains and artifacts from the soil.

A total of 51 lower jaws and 64 individual mammoth skulls were used to construct the walls, according to the investigation carried out by the team of researchers led by Dr. Alexander Pryor from the University of Exeter, UK.

In addition to the mammoth bones, the researchers were able to find evidence of charcoal and burnt bones, stone tool fragments, and soft plant tissue that hint at the diet consumed during those times.

Why would Pleistocene hunter-gatherers bother to erect such massive and, quite frankly, creepy structures?

“Kostenki 11 represents a rare example of Palaeolithic hunter-gatherers living on in this harsh environment. What might have brought ancient hunter gatherers to this site? One possibility is that the mammoths and humans could have come to the area on masse because it had a natural spring that would have provided unfrozen liquid water throughout the winter – rare in this period of extreme cold,” Pryor said in a statement.

The structure might have served as a dwelling for a small tribe or as a food stockpiling warehouse. The charcoal, for instance, suggests that fires were started inside the circular structure, providing solace against the harsh ice age nights. Climate modeling indicates that around the time the structure was erected, the last ice may have been at its worst, with temperatures around -20 degrees Celsius or lower.

Many of the bones were likely scavenged and transported to the site. Other bones likely came from hunting parties, with chunks of meat and tissue still attached to bones. Whatever their origin, a great deal of labor and planning was involved in order to transport such heavy loads.

The bones themselves don’t show signs of butchery. However, in the case of game of this size, the hunters probably removed the bulk of the meat, leaving small chunks to rot on the bone. Pryor says that humans butchering elephants in modern times using metal knives also didn’t leave any marks on the bones.

“These finds shed new light on the purpose of these mysterious sites. Archaeology is showing us more about how our ancestors survived in this desperately cold and hostile environment at the climax of the last ice age. Most other places at similar latitudes in Europe had been abandoned by this time, but these groups had managed to adapt to find food, shelter and water,” Pryor said.

However, Pryor writes that the amount of evidence that might point to intense activity at Kostenki 11 is rather low for what one might expect to find from a long-term base camp. He also has difficulty imagining how humans with limited technology could have been able to build the roof for such a large area, casting doubt on the site’s main use as a dwelling.

The structure perhaps also possessed a ritualistic significance. The Russian researchers speculate that it may have served as a shrine or monument honoring woolly mammoths. There is no evidence to back this assertion, which remains speculation at this point.

Whatever may be the case, this impressive archeological treasure trove shows that ice age humans were a lot more crafty than one might expect — after all, they had to in order to survive their extreme environment.

The findings appeared today in the journal Antiquity.