Current population: 20.5 million
The Mexican capital's population has grown extraordinarily over the past six decades. In 1950, the city had a population of just 2.9 million — making it the 16th largest in the world. But the population of Mexico City quadrupled over 30 years and has continued to expand ever since, with the current population at 20.5 million. The breakneck pace of growth has caused suffocating air pollution. Recent initiatives, including a World Bank–sponsored pollution-reduction program, have improved conditions, but smog persists.