Just a river’s crossing away from St. Louis, Missouri, rests an ancient and mysterious anthropological site that few Americans know of. Scholars still discuss the potential reasons for the demise of Cahokia, a massive settlement that may have housed as many as 20,000 people by 1050 A.D. The metropolis, which sits in the fertile floodplain of the Mississippi River Valley that’s now western Illinois, was made up of towering, handmade earthen mounds, the largest of which still exists at the Cahokia Mounds State Historic Site. While there are a lot of unknowns when it comes to this ancient civilization, including why it disappeared, remains have helped researchers paint a picture of what the city was like at its peak.
Read more: The Biggest Ancient City You’ve Probably Never Heard of Is in Illinois
