Cahokia Mounds is the area of an ancient Native American city close to Collinsville, Illinois. It is also across the Mississippi River from St. Louis, Missouri. The site, which has an area of nearly 4 square miles, includes about 90 earthwork mounds. The site is the largest example of Native American "civilization" in North America which was active beginning about 1000AD
It is a National Historic Landmark. In addition, it is one of only twenty one World Heritage Sites in the territory of the United States. It is the largest pre-Columbian construction in the United States.