Cahokia was the largest pre-Columbian city north of Mexico, with a population estimated at 10,000–20,000 at its peak (c. 1050–1200 AD). Monk's Mound required an estimated 22 million cubic feet of earth moved by hand over generations. The site includes a series of wooden post circles ('Woodhenge') that do function as solar calendars. Cahokia's decline was likely caused by environmental degradation, flooding, drought, and political fragmentation — documented through tree ring data and archaeological stratigraphy.