Cahokia

Around 1100 AD the largest city north of Mexico stood here, across the river from modern St Louis: perhaps twenty thousand people, a planned grid, plazas, a timber solar calendar, and more than a hundred earthen mounds. Monks Mound is thirty metres high with a base larger than the Great Pyramid’s, built by hand, basket by basket. It was abandoned by about 1350 and nobody is certain why, with drought, flooding, deforestation and political collapse all argued. It is a UNESCO World Heritage site, and the fact that most Americans have never heard of it is its own kind of story.