Cahokia was once a city on the Mississippi bigger than the London of its day

Across the Mississippi River from modern St. Louis, a scatter of earthen mounds rises out of the Illinois floodplain, the last visible trace of a city that once held more residents than London. Around 1100 CE, this Native American metropolis, now called Cahokia, sprawled across roughly six square miles and organized thousands of people around open plazas, timber temples, and an earthen mound larger at its base than the Great Pyramid of Giza. For a stretch of the medieval period it was the largest urban settlement north of Mexico, though few people outside archaeology circles know its name today.

A Sudden Boom Around 1050 CE

Before the middle of the eleventh century, the American Bottom region held only scattered farming settlements of fifty to a hundred people, most occupied for just five to ten years before residents moved on. Then, in what researchers call Cahokia’s “Big Bang,” three connected urban districts, later labeled St. Louis, East St. Louis, and Cahokia proper, went up almost simultaneously around 1050 CE. A new city grid took shape around the Grand Plaza and a causeway aligned with rattlesnake iconography, and pottery and building styles across the wider region suddenly grew uniform, a sign of large-scale immigration into the new city. A 2007 study in the journal Quaternary Science Reviews estimated that the high-density core grew from as few as 1,400 people in 1050 to more than 15,000 within fifty years.

A Population Rivaling London and Paris

Estimates of Cahokia’s total population at its peak vary widely, from roughly 6,000 up to 40,000 people, depending on how much of the surrounding farmland and satellite villages get counted alongside the urban core. If the higher estimates hold, Cahokia would have been larger than any city built later in what is now the United States until Philadelphia’s population passed 40,000 in the 1780s, and its twelfth-century population may have outnumbered contemporary London and Paris. Reaching that scale required moving an extraordinary amount of earth: workers dug and hauled roughly 55 million cubic feet of soil using nothing but woven baskets, raising around 120 mounds across the urban landscape. The largest of them, known today as Monks Mound, remains the largest earthen structure built anywhere in the Americas north of Mexico.

Plazas, a Timber Henge, and Public Ritual

Cahokia’s builders laid out the city with the same deliberateness they applied to its mounds. The Grand Plaza served as a flattened ceremonial ground bordered by causeways and residential mounds, while a separate ring of large timber posts, reconstructed today as Cahokia Woodhenge, appears to have tracked solstices and other solar events. A satellite site called the Emerald Acropolis, located in the surrounding uplands, was aligned instead to the moon’s roughly nineteen-year cycle and seems to have drawn pilgrims devoted to lunar and fertility rites. Archaeologists have also documented far grimmer ceremonies, including the burial of dozens of sacrificed women alongside powerful leaders at a location known as Mound 72, evidence of a rigid and highly ritualized social order.

Named After a Tribe That Arrived Later

Nobody knows what the city’s original builders called their home, because they left no written language behind beyond symbols pressed into pottery, shell, and copper. The name Cahokia comes from a historic Illiniwek people, the Cahokia tribe, who were living in the area when French explorers arrived in the seventeenth century, centuries after the Mississippian city had already been abandoned. There is no confirmed line of descent between that later tribe and the people who built the mounds, and archaeologists believe multiple indigenous groups likely occupied the area at different points across its history.

Why the City Emptied Out

Cahokia’s decline started in the late twelfth century. Around 1160 to 1170 CE, a walled residential compound in the East St. Louis district burned down along with ritual buildings packed with tools and stores of shelled maize, a fire some researchers read as a sign of internal unrest. Shortly after, around 1175 CE, residents built the first version of a defensive wooden palisade around the city’s ceremonial core, a wall that was rebuilt several times to enclose an ever-shrinking footprint as the population fell. By the middle of the thirteenth century the city had lost at least half its people, and it was fully abandoned by around 1350. Researchers have proposed a mix of explanations, including prolonged droughts between 1100 and 1250, health problems linked to heavy reliance on maize that had stopped being processed in a way that preserved its nutrients, and friction between long-established residents and the large immigrant population that had fueled the city’s earlier growth. Evidence of burned villages and defensive stockades elsewhere in the region has also fed theories involving outside conflict, though more recent flood and erosion studies have undercut older claims that Cahokians degraded their own environment. The abandonment was not unique to Cahokia: a much larger swath of the mid-continent, later dubbed the Vacant Quarter, emptied out at roughly the same time.

Recognition Centuries Later

The mounds survived under farmland and, later, under fill dirt taken to build parts of St. Louis, before growing archaeological interest led to formal protection. The site became a National Historic Landmark in 1964, was added to the National Register of Historic Places in 1966, and was named a UNESCO World Heritage Site in 1982. Its modern interpretive center, which opened in 1989, now draws up to a million visitors a year, and the Osage Nation, which traces its heritage to the region, works directly with site managers and has purchased a related mound across the river in St. Louis to preserve it. Further detail on the site’s excavation history is available through Wikipedia’s entry on Cahokia.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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