Over a thousand years ago, a sprawling city in the heart of modern-day Illinois rivaled London in size and complexity. Known today as Cahokia, it was home to around 15,000 people and served as the epicenter of the Mississippian culture—a pre-Columbian civilization that thrived across much of the Midwest and Southeast United States.

Cahokia was a marvel of its time. Its strategic location on the Mississippi River floodplains provided fertile land and access to vital trade routes, making it a hub for commerce, governance, and religious activities. The city boasted more than 100 monumental earthen mounds, with the towering Monks Mound as its centerpiece. This structure alone required an estimated 15 million basket-loads of soil to construct, and at its peak, a temple perched on its summit soared 50-75 feet above the mound.

The city was meticulously planned, featuring distinct neighborhoods, ceremonial plazas, and a remarkable “Woodhenge,” a circular arrangement of wooden posts aligned with the sun’s movements during solstices and equinoxes. These features demonstrate the advanced urban planning and astronomical knowledge of Cahokia’s builders, who thrived long before European settlers set foot in the Americas.

Despite its grandeur, Cahokia’s dominance was fleeting. By 1250, its population began to dwindle, and new construction ceased. Unlike many narratives of Native American decline, Cahokia’s fall wasn’t caused by European colonization or introduced diseases—it began over 200 years before Columbus.

The exact reasons remain a mystery, but theories abound. Environmental factors, such as droughts or crop failures, may have made sustaining the city impossible. Archaeological evidence suggests political unrest or conflicts with neighboring tribes could have destabilized its social fabric. Alternatively, Cahokia’s decline might have been a gradual process, as residents dispersed in search of better opportunities elsewhere.

While much of the ancient city has vanished—its mounds flattened during the development of nearby St. Louis—the remnants of Cahokia are now preserved as a UNESCO World Heritage Site. Visitors can walk among the surviving mounds, including the majestic Monks Mound, and imagine the vibrant community that once thrived there.

Modern technology is helping to uncover more of Cahokia’s secrets. Non-invasive techniques like magnetometry allow archaeologists to map buried structures without disturbing the ground. These studies are shedding light on Cahokia’s urban design, revealing a complex and well-organized society.

For more insights into this fascinating civilization, check out PBS’s video on What Happened to America’s First Megacity.

[PBS Terra]