The Mississippian people who built Cahokia between AD 1000 and 1350 moved an estimated 55 million cubic feet of earth by hand, basket by basket, to construct about 120 mounds across their city. The grandest of these is Monks Mound, which rises roughly 100 feet above the surrounding plain and covers 14 acres at its base, making it the largest earthen structure in the Americas. A massive building once stood at its summit, likely the residence of Cahokia's paramount chief, overlooking a Grand Plaza that stretched nearly 50 acres. Surrounding the ceremonial core, a wooden palisade wall nearly two miles long encircled the most important mounds and plazas. Reconstructions of this stockade now stand where archaeologists identified the original post holes, offering a tangible sense of the scale of Cahokian engineering.

West of Monks Mound, archaeologist Warren Wittry discovered in the early 1960s that a series of large post holes formed deliberate circles. He called these structures Woodhenges, comparing them to England's Stonehenge. Five separate timber circles were eventually identified, each larger than its predecessor, built in sequence between 900 and 1100 CE. From a central observation post, specific poles aligned with the sun's position at the solstices and equinoxes, creating what amounted to a monumental calendar. One reconstructed circle now stands at its original location, and visitors gather on mornings nearest the equinox and solstice to watch the sun rise over designated marker posts, just as Cahokian astronomers did a thousand years ago.

Cahokia emerged rapidly around AD 1050, during what archaeologists call the Big Bang, a sudden population explosion that transformed scattered villages into a planned urban center. The city drew people from across the region, becoming a melting pot of different Native American groups. Corn agriculture, long-distance trade networks stretching from the Great Lakes to the Gulf of Mexico, and a complex social hierarchy sustained the population for nearly three centuries. Then, by the 1300s, the city was abandoned. No single cause has been proven, though theories include environmental degradation, political upheaval, drought, and warfare. When French missionaries arrived in the 1600s, they found the mounds but no memory among local peoples of who had built them.

Even as farming and suburban sprawl consumed the American Bottom floodplain, the sheer scale of Monks Mound resisted erasure. Early 1800s historians recognized something extraordinary, but the mounds were dismissed by some as natural formations. It took Warren K. Moorehead's excavations in the 1920s to prove definitively that the mounds were human-made, prompting Illinois to purchase an initial 144 acres including Monks Mound. The site was designated a National Historic Landmark in 1964 and a UNESCO World Heritage Site in 1982, one of only a handful of such sites in the United States not managed by the federal government. Today the 2,200-acre site and its Interpretive Center tell the story of a civilization that built one of the great cities of the ancient world, just eight miles from downtown St. Louis.

The experience of Cahokia is remarkably personal. You climb the 156 steps to the top of Monks Mound and look south across the Grand Plaza, imagining the thousands who once gathered there for ceremonies, games, and markets. Deer wander through small forested patches between mounds. The Interpretive Center houses artifacts and a life-size reconstruction of a Cahokian neighborhood. Three self-guided trails lead past mounds of different shapes and purposes, from flat-topped platform mounds that once supported important buildings to conical burial mounds. Admission to the site is free. It sits in the Mississippi floodplain just off Interstate 55, close enough to St. Louis that the Gateway Arch is visible from Monks Mound, a modern monument to westward expansion overlooking the remains of a city that thrived centuries before the concept of westward expansion existed.