Cahokia was the largest city in North America until Philadelphia surpassed it in the 1780s. At its peak around 1100 AD, approximately 20,000 people lived in a planned urban center across the Mississippi River from present-day St. Louis. They built 120 earthen mounds, including Monks Mound - a 100-foot pyramid covering 14 acres, larger at its base than the Great Pyramid of Giza. They created a sophisticated society with social hierarchy, long-distance trade, astronomical observations, and organized agriculture. They had no writing system, so their history remains largely archaeological. By 1400, Cahokia was abandoned, its population dispersed for reasons that remain debated: environmental degradation, political upheaval, climate change, or some combination. The mounds remained, farmed over, quarried for fill, and largely ignored until archaeologists recognized what they represented. Now it's a UNESCO World Heritage Site - the forgotten city that challenges assumptions about pre-Columbian North America.
Cahokia was planned and built rapidly around 1050 AD, suggesting a single organizing authority. The city center featured Monks Mound (a four-terraced pyramid with buildings atop it), surrounded by a 2-mile-long wooden palisade. Within the palisade were plazas, additional mounds (platform mounds for temples and conical mounds for burials), and elite residences. Beyond were neighborhoods of ordinary houses, workshops, and agricultural fields. A series of wooden posts called Woodhenge served as a solar calendar. The city drew population from surrounding areas, creating suburbs that extended for miles.
Cahokia's society was stratified. The elite lived on or near mounds; commoners lived in outlying neighborhoods. Evidence suggests a paramount chief controlled the city, supported by a class of nobles. Human sacrifice occurred - one burial mound contained 53 young women, apparently killed together. Long-distance trade brought copper from Lake Superior, shells from the Gulf Coast, and mica from the Appalachians. The Cahokians were part of what archaeologists call the Mississippian culture, which spread across the eastern United States and shared characteristics including mound building, maize agriculture, and certain artistic motifs.
Cahokia's population peaked around 1100 and declined steadily thereafter. By 1300, most people had left; by 1400, the site was abandoned. The reasons remain debated. Environmental degradation is one theory - 20,000 people using wood for construction and fuel could have stripped the forest. The Little Ice Age brought climate stress. Political instability may have caused collapse. Flood damage appears in the archaeological record. Disease (possibly introduced through trade) is possible. The people didn't disappear - they dispersed to other settlements - but the city that had organized them was gone.
Europeans found the mounds in the 1600s but didn't connect them to indigenous peoples. Wild theories attributed them to Vikings, Phoenicians, or lost civilizations. Some mounds were quarried for road fill or flattened for farming. Monks Mound got its name from Trappist monks who gardened there briefly. Serious archaeology began in the 20th century. The site became a state historic site in 1979 and a UNESCO World Heritage Site in 1982. Modern research continues to reveal Cahokia's complexity - and the tragedy of how much has been lost.
Cahokia Mounds State Historic Site is located in Collinsville, Illinois, about 8 miles east of downtown St. Louis. The Interpretive Center provides excellent background on the Mississippian culture and Cahokia's history. A trail system connects major mounds; climbing the 154 steps to Monks Mound's summit offers views of the site and St. Louis skyline. Woodhenge is reconstructed. The Grand Plaza, once the city's center, is now grassy open space. The site is free and open daily. Allow at least 2-3 hours; serious exploration requires longer. St. Louis Lambert International Airport (STL) is 10 miles west. The site is most evocative on quiet weekday mornings.
Located at 38.66°N, 90.06°W in the Mississippi River floodplain in southwestern Illinois, 8 miles east of downtown St. Louis. From altitude, Monks Mound is visible as a large rectangular earthwork, the largest pre-Columbian structure north of Mexico. Additional mounds dot the surrounding fields. The Gateway Arch and St. Louis skyline are visible to the west across the Mississippi River. Much of the original site has been developed; the preserved area appears as green space amid industrial and suburban development.