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archaeological site occupying some 5 square miles (13 square km) on the Mississippi River floodplain opposite St. Louis, Missouri, near Cahokia and Collinsville, southwestern Illinois, U.S. The site originally consisted of about 120 mounds spread over 6 square miles (16 square km), but some of the mounds and other ancient features have been destroyed. Some 70 mounds are preserved in Cahokia Mounds State Historic Site. Established in 1979 and encompassing 3.4 square miles (8.9 square km), it was designated a UNESCO World Heritage site in 1982.