The Cahokia Settlement

The Atlantic Cities:

Archaeologists believe people began to gather at Cahokia around the year 1000 A.D. Inspired perhaps by the sighting of Halley's Comet in the year 989, settlers erected ceremonial mounds at the site, some of which line up with the sun's position during the winter solstice. Around the year 1100 they began to build Monks Mound — the largest mound, reaching some 100 feet off the ground, created from millions of baskets of dirt. A vast palisade that enclosed Monks Mound and other parts of the settlement was constructed around the year 1200. For reasons still debated, the whole city failed around the start of the following century.
The latest excavations have uncovered evidence of more than five hundred thatched houses and signs of workshops where residents created various goods. The homes surrounded the ceremonial sites, and at its peak the settlement may have expanded out into a primitive metropolitan area that served as residence to tens of thousands of Native Americans. But as a city Cahokia lacked the density of Mayan or European settlements; instead it appears to have organized itself more along the lines of "modern American urban sprawl," Lawler writes.
While settlement at Cahokia was short-lived, its cultural impact appears to have been widespread. Researchers working as far away as Wisconsin have found evidence of Cahokia-style pottery and housing. Why exactly the city disappeared it still a matter of conjecture. The leading assumptions point to political problems, a changing climate, or a combination of both. In a paper published in a 2009 issue of the journal American Antiquity, a research team led by Larry Benson of the U.S. Geological Survey presented climate-related evidence that "a series of persistent droughts occurred in the Cahokian area" which may have contributed to the city's abandonment:

By A.D. 1150, in the latter part of a severe 15-year drought, the Richland farming complex was mostly abandoned, eliminating an integral part of Cahokia's agricultural base. At about the same time, a 20,000-log palisade was erected around Monks Mound and the Grand Plaza, indicating increased social unrest. During this time, people began exiting Cahokia and, by the end of the Stirling phase (A.D. 1200), Cahokia's population had decreased by about 50 percent and by A.D. 1350, Cahokia and much of the central Mississippi valley had been abandoned.

Makes me think of the Dust Bowl.