#6: Cahokia (Illinois, USA)
North America’s greatest pre-Columbian city, Cahokia, thrived around 1100 CE near modern St. Louis. Its vast mounds and plazas suggest a complex society of tens of thousands. Yet by 1350, it was abandoned. Scholars suspect environmental stress or social upheaval. Today, its earthen remnants are silent witnesses to a lost metropolis.
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