A thousand years before Columbus's men would land on the shores of America, a new city was sprouting on the floodplains of modern-day St. Louis, Missouri—one that would become, within the course of just five decades, the the largest city ever built north of Mexico. The city of Cahokia, at its peak between the 10th and 12th centuries, was spread over 16 square kilometers—puny by today’s standards, but at that time it was larger than many European cities, including London. Cahokia boasted a population between 10,000 and 20,000 people and had at least 120 monumental earthen mounds that acted as foundations for many great buildings. Today, fewer than 80 mounds survive. How and why Cahokia declined remains one of America's great mysteries.

Monk’s Mound, the centerpiece of Cahokia Mounds State Historic Site. Photo credit: The Guardian