Capital city of Mexico. The largest city in the world, Mexico City boasts well over 20 million people. One of the oldest, if not the oldest, continuously occupied cities in the Americas; Mexico City grapples with innumerable problems including serious pollution. Originally a large lake, modern Mexico City engulfs what were once the Aztec cities of Tenochtitlan, Tlatelolco, Tacuba, and Xochimilco — today the names of neighborhoods of the sprawling giant. A cosmopolitan, bustling capital, Mexico city is today one of the world's great cities alongside Paris, New York, São Paulo, and Tokyo.