For 900 years, Moenjodaro, a city in what is now Pakistan, was the urban hub of a thriving civilization, the New York or London of its day. Around 1700 B.C., residents suddenly abandoned the Indus Valley city, and it was lost in the sands of time until archaeologists began excavating it in the 1920s. Today, visitors can wander for hundreds of acres among its deserted streets and homes.It's believed that Moenjodaro had already fallen into economic decline when an invading army attacked, delivering the sudden fatal blow. Moenjodaro never rose again, and the Indus Valley civilization that it dominated soon disappeared too.
Most of today's cities seem pretty sturdy. Indeed, the possibility that they might crumble to dust seems to be less of a concern than how nations will cope with the rise of so-called "megacities," cities with populations of more than 10 million: Tokyo, New York, São Paolo and Mumbai are already around twice that size or bigger.
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