Cahokia

The Cahokia Mounds is the site of a Native American city directly across the Mississippi River from present-day St. Louis. The state archaeology park lies in south-western Illinois between East St. Louis and Collinsville. The park covers 2,200 acres (890 ha), or about 3.5 square miles (9 km²), and contains about 80 manmade mounds, but the ancient city was much larger. At its apex around 1100 AD, the city covered about 6 square miles (16 km²), included about 120 earthworks in a wide range of sizes, shapes, and functions, and had a population of between 15,000 and 20,000 people.