About a thousand years ago, a city grew in the floodplain known as the American Bottom, just east of what is now St. Louis in Illinois. In a matter of decades, it became the continent’s largest population center north of Mexico, with perhaps 15,000 people in the city proper and twice as many people in surrounding areas. A couple centuries after its birth it went into decline, and by 1400 it was deserted.

The story of Cahokia has mystified archaeologists ever since they laid eyes on its earthen mounds—scores of them, including a 10-story platform mound that until 1867 was the tallest manmade structure in the United States. They don’t know why Cahokia formed, why it grew so powerful, or why