The pre-Columbian Mississippian culture and its capital at Cahokia (near modern St. Louis) challenges every assumption about what ancient North American societies were capable of. At its peak around 1100 AD, Cahokia had a population estimated at 10,000-20,000 people — comparable to contemporary London — with monumental earthworks, organized public space, and evidence of long-distance trade.

Monks Mound, the largest pre-Columbian earthwork in North America, contains more material by volume than the Great Pyramid of Giza. It was the centerpiece of a planned urban complex with a central plaza, satellite mounds, wooden palisade, and residential areas arranged around the monumental core.

Cahokia's political structure is not well understood. There are signs of hierarchical organization — different-quality housing, elaborate elite burials with mass human sacrifice — but no writing, no coins, no direct historical sources. The archaeological evidence suggests a complex chiefdom or early state whose exact organization remains contested.

Cahokia declined from around 1200 AD and was essentially abandoned by 1400. The causes — climate change, environmental degradation from intensive agriculture, political fragmentation, or some combination — are debated. The site was largely forgotten by European contact; De Soto's expedition in the 1540s found the region sparsely populated. The scale of what had been there was not recognized until modern archaeological survey.