Cahokia is the largest thing pre-Columbian North America built, and one of the least explained. For a few centuries it was bigger than London.
The rise. Something happened around 1050 that archaeologists call the "big bang": a scattered farming population was rapidly reorganised into a planned city with a grand plaza, aligned mounds and a rebuilt landscape. It was deliberate and fast, and it implies an authority able to command labour on an enormous scale. Monks Mound alone required tens of millions of basket-loads of earth carried by hand.
Mound 72. The most-discussed excavation uncovered a sequence of burials that speak to that authority. A high-status individual was laid on a platform of more than twenty thousand marine-shell beads, with other rich burials nearby, and separate pits held the bodies of dozens of young women, and of men who appear to have been executed. It is the clearest evidence of social hierarchy and of human sacrifice north of Mexico, and it is read carefully: the ideology that made sense of it is gone, and cannot be reconstructed from the bones.
The emptying. By about 1350 the city was effectively abandoned, before any European saw it. No single cause has won out: overhunting and deforestation of the floodplain, flooding, drought, internal political failure and out-migration have all been argued, and several may have acted together. What is not in doubt is that this was not a mysterious disappearance of a people but a dispersal: the descendants moved, and their nations exist. The name "Cahokia" is itself a later borrowing, from an Illinois-confederacy group who happened to live nearby when the French arrived, centuries after the mound builders had gone.