Cahokia, a Lost Metropolis

Cahokia, located just across the Mississippi River from present-day St. Louis in southwestern Illinois, was the largest pre-Columbian city north of Mexico. At its peak around 1100 AD, the city covered six square miles and was home to an estimated 20,000 people, a population larger than London at the time. Its most remarkable feature, Monks Mound, is a massive earthen pyramid larger at its base than the Great Pyramid of Giza. Cahokia was abandoned by 1350 AD for reasons that remain debated among archaeologists.

Source: Wikipedia