Managed by the Illinois Historic Preservation Agency, Cahokia Mounds State Historic Site, in Collinsville, Illinois, is located on the Mississippi River floodplain, across from St. Louis, Missouri. This site was first inhabited by Indians of the Late Woodland culture about AD 700. The site grew during the following Mississippian period, after AD 900, and by AD 1050-1150, the Cahokia site was the regional center for the Mississippian culture with many satellite communities, villages and farmsteads around it. After AD 1200, the population began to decline and the site was abandoned by AD 1400. In the late 1600s, the Cahokia Indians (of the Illinois confederacy) came to the area and it is from them that the site derives its name.

However, it is the building accomplishments and cultural developments of the earlier Indians that make this site significant. They constructed more than 120 earthen mounds over an area of six square miles, although only 80 survive today. These industrious people moved over an estimated 55 million cubic feet of earth in woven baskets to create this network of mounds and community plazas. Monks Mound, for example, covers 14 acres, rises 100 feet, and was topped by a massive 5,000 square-foot building another 50 feet high. As the largest prehistoric earthen construction in the Americas, Monks Mound is a testament to the sophisticated engineering skills of these people. Additionally, they built several "Woodhenges," large post-circle monuments that appear to have been used as calendars, and they also constructed several defensive palisades nearly two-miles long around the central ceremonial precinct.