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.
But could the opposite problem occur? Could some of our cities vanish as thoroughly as Moenjodaro did?
It's hard enough to predict 9 years into the future, much less 90. But here's some worst-case scenarios of what could happen by the end of the 21st century.
We do know for a fact that 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.
Whether these cities disappear entirely, of course, is an open question. Take Detroit, Mich., where the population has decreased by more than half since 1950, and now equals about 910,000. It could shrink slowly but steadily for decades to come; unemployment inside the city is more than 13 percent. Even if it doesn't disappear, if trends hold, Detroit may be altered beyond recognition by 2100.
Venice, Italy, is under threat as well. Italy's city of canals has been sinking for about a millennium, but in the past century the pace has accelerated rapidly. Venice has sunk about 24 centimeters over the last 100 years. The government has an engineering plan to protect it from rising sea levels, but no one really knows if it will work.
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.
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."
© 2012 Forbes.com