Look across the Mississippi River toward Illinois, where the Cahokia Mounds preserve the largest pre-Columbian city north of Mexico. Monks Mound, the central platform at Cahokia, covers more ground at its base than the Great Pyramid at Giza. It was built entirely by hand, one basket of earth at a time, by a civilization with no metal tools, no wheels, and no draft animals.
At its peak around 1050 to 1150 CE, Cahokia supported a population of perhaps 20,000 souls — a staggering urban concentration that would not be matched in North America north of the Rio Grande until Philadelphia in the late 1700s. The city was laid out with a central plaza, a wooden palisade wall, and a series of mounds for elite residences, ceremonial activities, and astronomical observation. Then, over the course of the 13th and 14th centuries, the population declined.
Archaeologists debate the causes: drought, deforestation, political instability, disease. By the time Europeans arrived, the city was long abandoned. Across the river, Cahokia's mounds still rise from the floodplain — a monument to a civilization that most Americans have never heard of.