Kolkata
Predicted increase in population by 2025: 4.56 million
As India's third most populous city, located on the banks of the Hooghly River, Kolkata has long been a hub for trade, drawing migrants from neighboring states in search of employment. But surrounding marshes and swampland have confined the metropolitan area to a small space, packing in its inhabitants extra tightly, and the city's propensity to monsoons can lead to unfathomable flooding. A housing shortage and nightmarish traffic jams are serious problems. The number of vehicles in the city has grown drastically over the past few decades, but a new transportation system is under way to accommodate the large number of commuters. The city is working quickly to expand its underground-tram system, but it must persuade residents to use it — as an outdated bus system remains the most popular method of public transit.