Mexico City, Mexico, satellite image. Mexico City, located in southern central Mexico, is the country's capital and largest city. The city was built in the 16th century by the Spanish, on the ruins of the Aztec city of Tenochtitlan. Mexico City has a population of over 18 million people, making it one of the most populated cities in the world, and it is severely overcrowded. Much of the population lives in slums, with an estimated one third of the city's residents being without sewage facilities. Taken by the Ikonos satellite on 7 August 2003.