Dhaka, fastest growing city in the world
By 2025, the U.N. predicts Dhaka will be home to more than 20 million people -- larger than Mexico City, Beijing or Shanghai.
Among these megacities, The World Bank says Dhaka, with its current population of 15 million people, bears the distinction of being the fastest-growing in the world . Between 1990 and 2005, the city doubled in size -- from 6 to 12 million. By 2025, the U.N. predicts Dhaka will be home to more than 20 million people -- larger than Mexico City, Beijing or Shanghai.
Mass migration, booming populations and globalized trade are swelling cities worldwide, but these forces are perhaps more powerfully concentrated in Dhaka than anywhere on earth -- offering a unique window on an urban planet soon to come.
"You are seeing the early future of the world, which is not a very pleasant thought," said Atiq Rahman, a Dhaka climate and migration researcher who heads the Bangladesh Centre for Advanced Studies. Explosive growth in cities like Dhaka, he said, has created "a cluster of demographic chaos."
The earth's countryside is emptying out, more quickly all the time. It took about 10,000 years  for the human population to become 3 percent urban  -- a period extending roughly from the dawn of human settlement until 1800. A century later , Earth was still just 14 percent urban. But in 2007, the United Nations announced we'd crossed a monumental threshold. For the first time, more than 50 percent of the world lived in cities rather than rural villages and farms. By 2030, some projections  say more than 80 percent of humanity will be urban, with many inhabiting the slum-choked cities of the developing world.
The shift is "a watershed in human history, comparable to the Neolithic or Industrial revolutions," urban theorist Mike Davis wrote in his book "Planet of Slums."
In the simplest sense, this transformation has a dual cause: Masses of migrants are abandoning the countryside, and they keep having babies after coming to town. By some accounts, fertility is a larger slice of the pie.
"It's roughly a 40/60 split," said Deborah Balk, an urbanization specialist with the CUNY Institute for Demographic Research in New York City. "We have more large concentrations of people than we've ever had before. That is new. And those concentrations themselves, they have momentum."
As many as half of the newcomers  worldwide will end up in medium-sized cities with populations of up to half a million people, by the U.N.'s count. But many migrants will make their home in a growing number of megacities -- urban giants such as the world has never seen.
By 2025 the U.N. predicts that Delhi, Dhaka, Kolkata, Mumbai, Mexico City, New York, Sao Paulo and Shanghai will all have populations of more than 20 million. Tokyo is projected to become home to some 37 million -- three times the current population of Greece.