Cahokia

38.6534°N, 90.0623°W.

Mississippian pre-Columbian city across the river from modern St. Louis, occupied from approximately 1050 to 1350 CE with a peak population estimated at 15,000–40,000 — larger than London at the same date. The city contained at least 120 earthen mounds across 16 km²; Monks Mound, the largest, is a four-tiered platform 30 metres tall and 290 by 236 metres at the base, the largest pre-Columbian earthwork north of Mexico. Woodhenge, a series of five circular timber alignments built and rebuilt between 900 and 1100 CE, marked the equinoxes and solstices through sightlines from a central observation post. Mass burials at Mound 72 contain over 270 individuals, many sacrificed; one beaded burial of a high-status male is overlaid with 20,000 marine-shell beads in the shape of a falcon.

The city was abandoned by 1400 CE — over a century before European contact — and the people who built it never told anyone why they left.