Fifteen hundred years ago, Teotihuacan was one of the world's greatest cities. Some 200,000 people lived in this Mexican metropolis, with its massive public buildings, grid plan of streets and imposing murals and sculpture. Its trading empire dominated much of ancient Mexico. Then, in the 8th century, came a mysterious collapse. Even knowledge of the original name was lost: Teotihuacan, City of the Gods, was a title bestowed by the Aztecs six hundred years later.