Cahokia

Across the river from where St. Louis stands today, the Mississippian people built Cahokia, the largest city in North America north of Mexico — a metropolis of perhaps 20,000 centered on enormous flat-topped earthen pyramids and a plaza aligned to the heavens. For a time it was larger than London. Then, over the 1300s, it was abandoned, likely amid climate stress and political strain — proof that the Americas held cities and states long before any European arrived to 'discover' them.