Cahokia is the ancient Illinois city known for North America's largest pre-Columbian earthwork. Located near present-day Collinsville, across the Mississippi River from St. Louis, Cahokia was the largest urban center of the Mississippian culture.
The city flourished roughly from 1050 to 1350 CE and may have supported 15,000–20,000 people at its peak. Its inhabitants built plazas, ceremonial structures, wooden circles called Woodhenges, and about 120 earthen mounds. Monks Mound, the site's central platform mound, is the largest of these structures.
Monks Mound rises about 100 feet and covers roughly 14 acres. It was made from layers of soil and clay carried in baskets, not from cut stone like an Egyptian pyramid. Construction took place in stages, probably over several centuries, and the summit once supported a large wooden building.
A common mix-up is naming Monks Mound rather than Cahokia. Monks Mound is the specific earthwork; Cahokia is the ancient city and archaeological site that contains it. The Cahokia Mounds site became a UNESCO World Heritage Site in 1982.