Over Christmas I read an excellent article by a cultural anthropologist regarding Detroit; within the "city limits" something like a quarter of all business buildings are abandoned and close to half the homes.
The upthrust of the article was how anthropologists have long speculated on the ruins of some large and well established cities within the Pre-Colonial Americas appear to have simply ceased to be. Speculation runs everywhere from disease to warfare, but one that never gained too much weight was they collapsed from within due to their own "social" weight.
Some cultural anthropologists are studying what is going on in Detroit because it's giving them insights into what may very well have happened to those lost city-states. Needless to say, they never expected to see it happen in their life times without some sort of cataclysmic event.