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Cahokia Mounds
The largest city in North America north of Mexico, around 1100
Around the year 1100, the largest city in what is now the United States stood on the Mississippi River floodplain across from present-day St. Louis. Cahokia, built by the Mississippian culture, covered about 6 square miles (16 km²), held some 120 earthen mounds, and may have been home to 15,000 to 20,000 people, more than London at the time. Its centerpiece, Monks Mound, rises about 100 feet (30 m) and covers nearly 14 acres at its base, the largest prehistoric earthwork in the Americas north of Mexico.
Cahokia's builders laid out great plazas, a wooden stockade and circles of tall posts, nicknamed Woodhenge, that tracked the rising sun through the year. The city peaked around 1100 and was largely abandoned by about 1350, for reasons archaeologists still debate. About 80 mounds survive inside Cahokia Mounds State Historic Site, run by the State of Illinois. It became a UNESCO World Heritage Site in 1982, one of only a handful of American sites honored for their Native history.