North of St. Louis, across the Mississippi River in present-day Illinois, Cahokia Mounds preserves the remains of a city that once reorganized life across a broad region. UNESCO dates its principal occupation to the Mississippian period, from about 800 to 1350, and describes Cahokia as the largest pre-Columbian settlement north of Mexico. The fields, low ridges and immense earthen platforms can seem quiet today; at their height they formed a connected urban landscape, not a collection of isolated burial mounds.
The scale is striking but should be read as an archaeological estimate, not a precise census. UNESCO says the settlement once covered more than 1,600 hectares and included about 120 mounds; its agricultural society may have supported 10,000 to 20,000 people at its peak, between roughly 1050 and 1150. Those figures are estimates assembled from evidence, and the boundary of the ancient settlement is not the same as the visitor paths open today. They convey the exceptional concentration of homes, public spaces and earthworks that made Cahokia a major center.