Cahokia is the site of an ancient Native American city near Collinsville, Illinois, across the Mississippi River from St. Louis, Missouri in the American Bottom flooplain. The site is composed of a series of man-made earthen mounds. Cahokia is the largest archaeological site related to the Mississippian culture, and the term "Cahokian" is sometimes used to describe that culture. The Mississippians developed advanced societies in eastern North America before the arrival of Europeans.