Little is known about the people who built this huge metropolis, occupied it for a thousand years, and then abandoned it around 850 A.D., five hundred years before the Aztecs arrived. In 1963 alone, Mexico spent more than a million dollars to probe its secrets. In this article, DR. IGNACIO BERNAL, one of Mexico’s leading archaeologists and the director of the new museum at Teotihuacan, describes the work of excavation and restoration.