Archaeologists have found plenty of structures made of mammoth bones across Eastern Europe over the years, but none quite like this one. Previously discovered ones were fairly small, suggesting they were used as dwellings. But this is not only older than the rest—figure about 25,000 years old—it's also much bigger, reports Haaretz. At about 40 feet wide, archaeologists estimate it took the bones of at least 60 woolly mammoths to complete the circular structure, per Smithsonian. These bones are massive and heavy, making this no small feat. "The sheer number of bones that our Paleolithic ancestors had sourced from somewhere and brought to this particular location to build this monument is really quite staggering," Alexander Pryor, an archaeologist at England's University of Exeter, tells the New York Times. "It does boggle my mind."
The structure, found on the Russian plains about 300 miles south of Moscow, was likely too big for a roof, meaning it probably wasn't a dwelling. One theory is that it served as a sort of butchery. Researchers found the remains of burnt wood and burnt bones inside the structure, and the Times speculates that hunter-gatherers may have started wood fires, then added greasy bones to make them burn brighter. That way, they could have worked into the night to strip carcasses of meat before predators arrived. Several pits nearby suggest the meat might have been stored there in frigid Ice Age temperatures. Still, the overall size of the structure suggests it wasn't merely functional. "Ritual is embedded in human lives in all sorts of ways," says Pryor, per Smithsonian. "The fact they might have designed a structure of this type as part of both their ritual and their sustenance activities is very reasonable." (Read more archaeology stories.)