Excavations at the city, famous for its pre-Columbian mounds, challenge the idea that residents destroyed the city through wood clearing. A thousand years ago, a city rose on the banks of the Mississippi River, near what eventually became the city of St. Louis. Sprawling over miles of rich farms, public plazas and earthen mounds, the city — known today as Cahokia — was a thriving hub of immigrants, lavish feasting and religious ceremony. At its peak in the 1100s, Cahokia housed 20,000 people, greater than contemporaneous Paris. By 1350, Cahokia had largely been abandoned, and why people left the city...