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For 900 years, Mohenjo-daro, 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 Mohenjo-daro had already fallen into economic decline when an invading army attacked, delivering the sudden fatal blow. Mohenjo-daro 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, Sao Paolo and Mumbai are already around twice that size or bigger.
But could the opposite problem occur? Could some of our cities vanish as thoroughly as Mohenjo-daro did?
It's hard to predict, of course, but factors as diverse as climate change and aging populations mean that even as the global urban population continues to grow, some cities are shrinking.
It's not just small towns, although in wealthy nations, small communities may face the most extreme effects. In Japan, many rural hamlets, left with only a few elderly residents, are in danger of total disappearance. In the U.S., towns in Kansas and the Dakotas face extinction mainly because of an exodus of young people. Some Kansas towns are fighting back by giving away free land, with mixed results.
But some bigger centers also face the risk of annihilation. Urban planners across Europe and North America are already grappling with what to do with "shrinking cities." After the fall of the Berlin Wall in 1989, millions of residents of what had been East Germany moved west. More than a million apartments were simply abandoned.
In response, the German government sponsored the Shrinking Cities Project to study what is now a global phenomenon. The project has an exhibit on tour that examines shrinkage in Russia's Ivanovo, Leipzig in Germany, Manchester and Liverpool in Britain and Detroit in the U.S.
Whether these cities disappear entirely, of course, is an open question. Detroit's population has fallen by around a third since 1950 and now equals about 950,000. It is expected to shrink slowly but steadily until at least 2030; unemployment inside the city is more than 10%. (The suburbs around Detroit, meanwhile, are growing.) If trends hold, Detroit will be altered beyond recognition by 2100.
As Detroit flirts with demographic disaster, some cities face the natural kind. Researchers at the University of California, Davis, forecast a 75% chance that San Francisco will be struck by a major earthquake of magnitude 7 or above by 2086.
Some might argue that city dwellers will survive and rebuild, although the fate of New Orleans in the wake of Hurricane Katrina, which flooded 80% of the city in 2005, offers mixed lessons.
San Francisco is also one of the fastest-shrinking cities in California, part of an overall population shift away from the expensive and geographically hazardous coast toward inland cities. A major disaster could accelerate that trend.
Rising sea levels threaten cities around the world. The industrious Dutch have strong enough dikes and clever enough engineering to survive a one meter rise in the oceans, even though two-thirds of their country lies below sea level.
But Banjul, capital of Gambia in West Africa, is likely to sink entirely into the ocean due to a combination of erosion and rising sea levels, according to a 2002 World Bank discussion paper on cities and climate change. The same paper forecasts that sea levels will rise between 10 and 90 centimeters worldwide this century, affecting many coastal cities, including Alexandria, Egypt; Tianjin, China; Jakarta, Indonesia; and Bangkok, Thailand.
Whether from natural catastrophes, economic collapse or the slow encroachments of sand or water, it seems likely that at least some of today's cities will meet the same fate as Ozymandias, the king of kings who built a monument to himself.
As the English poet Percy Bysshe Shelley wrote, "Round the decay / Of that colossal Wreck, boundless and bare / The lone and level sands stretch far away."