Growing up in St. Louis, I learned about Cahokia Mounds in school. It’s fascinating how little we know about the settlement that was once North America’s largest city. If you grew up outside Missouri or Illinois, you probably never heard about it at all.

A thousand years ago, huge pyramids and earthen mounds stood where East St. Louis sprawls today in Southern Illinois. This majestic urban architecture towered over the swampy Mississippi River floodplains, blotting out the region’s tiny villages. Beginning in the late 900s, word about the city spread throughout the southeast. Thousands of people visited for feasts and rituals, lured by the promise of a new kind of civilization. Many decided to stay.

They didn’t stay long though. By 1400, the city was abandoned and no one knows why. A group of archaeologists are attempting to unlock the mysteries of the Mississippian people. Check out the coverage on Ars Technica.