Cahokia is the site of an ancient indigenous city (ca 600–1400 CE) near Collinsville, Illinois. In the American Bottom floodplain, it is across the Mississippi River from St. Louis, Missouri. The 2,200-acre (8.9 km2) site included 120 man-made earthen mounds over an area of six square miles, although only 80 survive. Cahokia Mounds is the largest archaeological site related to the Mississippian culture, which developed advanced societies in eastern North America centuries before the arrival of Europeans.
It is a designated site for state protection and a National Historic Landmark. In addition, it is one of only twenty World Heritage Sites designated in the territory of the United States. It is the largest prehistoric earthen construction in the Americas north of Mexico.
Links: en.wikipedia.org, virtualglobetrotting.com