A thousand years ago, huge pyramids and earthen mounds stood where East St. Louis sprawls today in Southern Illinois. This majestic urban architecture towered over the swampy Mississippi River floodplains, blotting out the region's tiny villages. Beginning in the late 900s, word about the city spread throughout the southeast. Thousands of people visited for feasts and rituals, lured by the promise of a new kind of civilization. Many decided to stay.

At the city's apex in 1050, the population exploded to as many as 30,000 people. It was the largest pre-Columbian city in what became the United States, bigger than London or Paris at the time. Its colorful wooden homes and monuments rose along the eastern side of the Mississippi, eventually spreading across the river to St. Louis. One particularly magnificent structure, known today as Monk’s Mound, marked the center of downtown. It towered 30 meters over an enormous central plaza and had three dramatic ascending levels, each covered in ceremonial buildings. Standing on the highest level, a person speaking loudly could be heard all the way across the Grand Plaza below. Flanking Monk’s Mound to the west was a circle of tall wooden poles, dubbed Woodhenge, that marked the solstices.

Despite its greatness, the city’s name has been lost to time. Its culture is known simply as Mississippian. When Europeans explored Illinois in the 17th century, the city had been abandoned for hundreds of years. At that time, the region was inhabited by the Cahokia, a tribe from the Illinois Confederation. Europeans decided to name the ancient city after them, despite the fact that the Cahokia themselves claimed no connection to it.

Centuries later, Cahokia's meteoric rise and fall remain a mystery. It was booming in 1050, and by 1400 its population had disappeared, leaving behind a landscape completely geoengineered by human hands. Looking for clues about its history, archaeologists dig through the thick, wet, stubborn clay that Cahokians once used to construct their mounds. Buried beneath just a few feet of earth are millennia-old building foundations, trash pits, the cryptic remains of public rituals, and in some places, even, graves.

To find out what happened to Cahokia, I joined an archaeological dig there in July. It was led by two archaeologists who specialize in Cahokian history, Sarah Baires of Eastern Connecticut State University and Melissa Baltus of University of Toledo. They were assisted by Ph.D. candidate Elizabeth Watts of Indiana University, Bloomington, and a class of tireless undergraduates with the Institute for Field Research. Together, they spent the summer opening three large trenches in what they thought would be a sleepy little residential neighborhood southwest of Monk's Mound.

They were wrong. The more they dug, the more obvious it became that this was no ordinary place. The structures they excavated were full of ritual objects charred by sacred fires. We found the remains of feasts and a rare earthen structure lined with yellow soils. Baires, Baltus, and their team had accidentally stumbled on an archaeological treasure trove linked to the city's demise. The story of this place would take us back to the final decades of a great city whose social structure was undergoing a radical transformation.

East St. Louis palimpsest

Finding a lost city in the modern world isn’t exactly like playing Tomb Raider. Instead of hacking through jungle and fighting a dragon, I drove to Cahokia on a road that winds through the depressed neighborhoods of East St. Louis and into Collinsville, Illinois. As recently as the 1970s, the ancient city’s elevated walkways and mounds were covered over by suburban developments. Just west of Monk's Mound was the Mounds Drive-In Theater. Farmers often plowed over Cahokia’s smaller landmarks.

All that changed 40 years ago when Illinois declared Cahokia a state historic site, and UNESCO granted it World Heritage status. The state bought 2,200 acres of land from residents, clearing away the drive-in and a small subdivision. Now the Cahokia Mounds State Historic Site and Visitors’ Center is devoted to preserving what remains of the ancient city’s monumental downtown architecture.

When I arrived there last summer, archaeologists Baires, Baltus, and their team had already been digging for several weeks in the broiling southern Illinois heat. To reach their excavation, I pulled up on a gravel turnout behind some old gas tanks and trudged through the muddy grass of an unmarked field until I saw a bunch of people with shovels clustered around three open pits. It was 7am, but I was already a bit tardy—the team started every day around 6:30am to avoid working through the late afternoon heat.

Baires and Baltus chose to explore this unassuming area known as the CABB Tract based on a magnetometry survey that Watts had done several months before. Using a handy shoulder-mounted magnetometer, Watts carefully paced out the entire field, looking for signs of ancient habitation.

Magnetometers are perfect for sniffing out buried structures because they can detect anomalies that represent disturbed earth, burned objects, and metals several feet beneath the surface. Watts' magnetometry map revealed a distinctive pattern of promising dark rectangular spots, their shapes and positions too precise to be natural. They looked an awful lot like the floors of homes arranged in a semi-circle, perhaps around a courtyard.

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The courtyard shape is what caught Baltus and Baires’ attention. Late in Cahokia's history, there was an inexplicable shift in the city's layout: People abruptly stopped building on a north-south grid and returned to open courtyard plans that imitated the village layouts from before Cahokia's founding. The archaeologists wanted to know what ordinary people were doing during the city's transition, and this spot was well beyond the elite sphere of Monk's Mound. They broke into the earth above three separate anomalies, eventually creating three trenches called excavation blocks (EB 1, 2, and 3 for short).

When I arrived, Baires, Baltus, and Watts were looking down into EB1, muttering to each other about what they’d found. "Ugh—what is this?" Baires asked, looking intently at the floor of a structure that had not seen light for almost a thousand years. I knelt down next to her at the carefully squared-off edge of the pit, trying to imagine a building here. "It's a palimpsest,” Watts suggested. The group had uncovered layer upon layer of material, suggesting many structures were built in this same place over time. Like most of the team, Watts stood barefoot in the muddy trench so as not to disturb the ground where Cahokians once walked.

Even with my untrained eye, I could tell she was pointing at overlapping building floors: one area of darker clay ended abruptly in a diagonal line like a wall, and alongside it was a uniformly colored area of clay studded with charcoal and artifacts. The walls themselves, made from posts sunk into the clay, had long ago rotted away.

These structures weren’t modest little homes, either. At least one ritual fire had burned here, its flames consuming valuable offerings like mica, a ceremonial beaker for holding the heavily caffeinated Black Drink, a beautifully woven mat, a pottery trowel imported from a remote village, and an ancient projectile point from pre-Cahokia peoples that would have been centuries old by the time it was buried here. EB 2 and 3 were similarly unusual, yielding finds that suggested feasting and ritualistic earth-moving activities.

What Baires and Baltus thought would be a bunch of private homes turned out to be a public area full of “special use structures,” the preferred archaeological term for any building whose purpose goes beyond the everyday. People used these buildings for everything from political debates and social gatherings to spiritual practices and party venues. Looking over the neighborhood, Baires said simply, "I've never seen anything like this." Following her gaze, I could no longer see the field bordered by trees and distant gas tanks. Instead, there were meeting halls, a wide courtyard with a decorated wooden pole at its center, and a sacred pit where Cahokians borrowed clay for their mounds. A huge trash pile full of deer bones and broken pottery hinted at a big feast.

I was looking back in time to a period when the quiet fields around me would have been packed with people, houses, and mounds all the way to the horizon.