Projected Population in 2015: 21.6 million
Population in 2005: 19.4 million

Mexico's capital sprawls over the site of Tenochtitlan, capital of the Aztec Empire, which fell to the conquistador Hernan Cortes in 1521. A more recent disaster was the major earthquake that struck in 1985. This massive metropolis is Mexico's capital in every sense--political, financial and cultural. Built over a lake bed, the city is slowly sinking as it sucks water from the aquifer beneath it to slake the thirst of its teeming millions.