The Cahokia mounds are a group of prehistoric mounds, mostly in the form of truncated pyramids, near East St. Louis, Illinois. The largest of these, Monks Mound, is about 350 meters by 200 meters at the base, and some 30 meters high. It is the largest mound in the United States. More than 300 of the Cahokia mounds were bulldozed to make way for agricultural and municipal expansion, but the largest 18 remain.