This gnarled pile of fossils was once a giant structure—made of Ice Age mammoths

Some 25,000 years ago, a circular wall of bone and ivory rose like a macabre mirage from a snowbound plain 500 kilometers south of present-day Moscow. The ring—built from the bones of at least 60 mammoths—was thought to shelter people living on the treeless expanse during the coldest part of the last ice age. Now, a new study reveals the ring, discovered in 2014, is 12.5 meters in diameter—likely too large to have been roofed. Archaeologists also failed to find any remains from animals other than mammoths, making it unlikely that humans lived there for any length of time.

The ring, found at a site called Kostenki, is the oldest such structure found in Russia. It’s about 3000 years older than two similar, smaller mammoth bone rings found at the same site almost 40 years ago.

Although it’s not clear why nomadic hunter-gatherers would have built such a permanent, labor-intensive structure, most scientists assumed they assembled it from mammoth bones because the region had precious few trees during the ice age. So it came as a bit of a surprise when workers at the site sifted out hundreds of bits of charcoal, dated to about 25,000 years ago, from the soil.