Monks Mound is the largest earthen structure at Cahokia

Cahokia Mounds State Historic Site (pron.: /kəˈhoʊkiə/) is located on the site of an ancient Native American city (c. 600–1400 CE) situated directly across the Mississippi River from modern St. Louis, Missouri. This historic park lies in Southern Illinois between East St. Louis and Collinsville. The park, operated by the Illinois Historic Preservation Agency, is quite large, covering 2,200 acres (890 ha), or about 3.5 square miles, and containing about 80 mounds, but the ancient city was actually much larger. In its heyday, Cahokia covered about 6 square miles and included about 120 man-made earthen mounds in a wide range of sizes, shapes, and functions.^[3]

• http://en.wikipedia.org/wiki/Cahokia