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Cahokia becomes a major city
A huge Native American city near today's St. Louis
Cahokia, near present-day St. Louis, was the largest city north of Mexico in its time. Its centerpiece is Monks Mound, a huge earthen mound.
Cahokia was settled around 700 CE and flourished from about 950 to 1350, according to Britannica, with its greatest growth in the 11th and 12th centuries. It sat on the Mississippi floodplain in present-day Illinois, and it was the largest urban center in North America north of Mexico.
Monks Mound is about 100 feet tall and covers more than 14 acres. It was built in stages, with Britannica dating construction between roughly 900 and 1200. It is the largest prehistoric earthen structure in the Western Hemisphere.
Estimates of the peak population range from about 8,000 to 40,000, and a figure around 15,000 to 20,000 is common. The site was largely abandoned by about 1350, and scholars point to causes such as social unrest, environmental stress and an earthquake, without agreement on any single one.
The city was planned around a central plaza, and its builders also raised a ring of posts known as Woodhenge, which archaeologists think was used to mark the solstices and equinoxes. UNESCO added Cahokia Mounds to the World Heritage List in 1982. Archaeologists identify a sharp growth in settlement around 1050, which is the basis for the story's date range, though sources vary on how rapid it was.