The Cahokia Mounds State Historic Site is the site of a pre-Columbian Native American city directly across the Mississippi River from modern St. Louis, Missouri. This historic park lies in south-western Illinois between East St. Louis and Collinsville. The park covers 2,200 acres, or about 3.5 square miles, and contains about 80 mounds, but the ancient city was much larger. At its apex around 1100 CE, the city covered about 6 square miles and included about 120 manmade earthen mounds in a wide range of sizes, shapes, and functions. At the apex of its population, Cahokia may have briefly exceeded contemporaneous London, which at that time was approximately 14,000–18,000.
Cahokia was the largest and most influential urban settlement of the Mississippian culture, which developed advanced societies across much of what is now the central and southeastern United States, beginning more than 1,000 years before European contact. Today, the Cahokia Mounds are considered to be the largest and most complex archaeological site north of the great pre-Columbian cities in Mexico.
Cahokia Mounds is a National Historic Landmark and a designated site for state protection.