A city that was the largest in the Western Hemisphere by 500 AD and had already stood in ruins for centuries when the Aztecs arrived. They did not know who built it and named it the place where men become gods. Beneath the Temple of the Feathered Serpent, archaeologists found a tunnel in 2003 that had been sealed for nearly two thousand years: thousands of objects inside, chambers with ceilings dusted in pyrite to imitate a night sky, and pools of liquid mercury.