Does this sound in any way familiar?

"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."

Please re-read that carefully; the reference is to a pre-contact municipality in Illinois in the twelfth through fourteenth centuries, not to Detroit forty or so years ago -- or this week, for that matter.

There's more work being done these days on Cahokia, mostly due to some salvage archaeology related to bridge construction. It was a bigger deal, in its day, then we thought. The link is to a summary piece, since you have to be a subscriber to Science to read the full text.