Cahokia: what actually happened.

Around the year 1100, the largest city north of Mexico was not in Mexico at all. It was in what is now Illinois, across the Mississippi from modern St. Louis, and it was bigger than London.

Cahokia covered about six square miles and held somewhere between 15,000 and 20,000 people at its peak — a population comparable to twelfth-century London and Paris. Around 120 earthen mounds rose across the city in every size and shape, arranged around plazas, and about 80 of them are still there. The city grew with startling speed: between roughly 1050 and 1100, its population exploded from a couple of thousand to more than ten thousand, as people streamed in from across the midcontinent — a demographic detonation archaeologists sometimes call the “Big Bang.”

And at the center of it stood Monk’s Mound: four terraces, ten stories tall, the largest man-made earthen mound north of Mexico and the largest pre-Columbian earthen construction in the Americas. It was not raised in a single campaign. It went up over centuries, in as many as ten separate construction episodes — built taller, widened, terraced, re-faced — each generation adding its layer to the last. In total, the builders of Cahokia moved an estimated 55 million cubic feet of earth. They did it the only way available: by hand, in woven baskets, carried up the mound one load at a time. Picture that for a moment. Not a decade of weekends. Centuries of basket-loads, organized, fed, and directed, until a hill stood where no hill had been.

What was it for? The mound’s summit held large buildings — one excavated structure measured about 105 feet long and 48 wide, possibly 50 feet high — and the whole complex faced a 50-acre Grand Plaza, a packed-earth public square ringed by smaller mounds. This was a stage for ceremony and power: the elite literally elevated above the city, with the plaza below holding the crowds. Everything about Cahokia’s layout is deliberate — a planned city, raised by a society that could mobilize thousands of workers for generations without iron, without the wheel, without draft animals.

Then, around 1160 to 1170, the depopulation began. The reasons are still debated — climate shifts, resource strain, political fracture — but the city emptied over the following centuries, and by the time Europeans arrived the mounds were already ancient, their builders’ name for the place lost. What remains is the earthwork itself, which is the point: stone crumbles and wood rots, but 55 million cubic feet of packed earth is still there, holding the shape of an ambition most modern cities never attempt.

Cahokia is what actually happened: a thousand years ago, the American Bottom held a metropolis that stood shoulder to shoulder with the great cities of Europe, built one basket-load at a time — and then walked away from, leaving the biggest thing anyone had ever made out of dirt as the receipt. The mound is still growing in one sense: every excavation season refines the picture, and the population estimates keep climbing as new residential areas turn up west of the old boundaries. The city was bigger than we thought, and we are still catching up to it.