Ice Age hunter-gatherers, foraging the bone-chilling, unforgiving steppes of what today is Russia, somehow completed a remarkable construction project: a 40-foot-wide, circular structure made from the skulls, skeletons and tusks of more than 60 woolly mammoths. The reason remains a mystery to archaeologists.

“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,” said Alexander Pryor, an archaeologist at the University of Exeter in England. “It does boggle my mind.”

Alexander Dudin, a researcher from the Kostenki Museum-Preserve, and a team of scientists began excavating the 25,000-year-old mammoth-bone circle in 2014 at a site called Kostenki 11, which is 300 miles south of Moscow. It is the third structure uncovered at the site. The discovery was published Monday in the journal Antiquity.

Archaeologists have unearthed about 70 mammoth-bone structures across Eastern Europe. But this one is the oldest on the Russian plain thought to be made by modern humans. Most of the previously identified structures were small, leading researchers to conclude they were most likely used as winter dwellings on a nearly treeless landscape.