Mumbai (Bombay), India

Dharavi is a one-square-mile (three-square-kilometer) slum in the heart of bustling Mumbai, India's other megacity .

Human densities in the city have reached 18,000 people an acre, and a 300-square-foot (28-square-meter) dwelling is typically home to 15 people.

According to a new UN report, by 2025 two more Indian cities will join New Delhi and Mumbai as megacities: Kolkata (Calcutta) will reach 20.6 million inhabitants and Madras will host 10.1 million.

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