Teotihuacan

Cracked:

Just outside of Mexico City sits Teotihuacan, a vast ruined city of pyramids and palaces. Teotihuacan is not its original name, but rather one invented by the Aztecs, who gave it a title that would dislocate the tongue of anybody trying to speak it in order to hide it from the civilized world. You see, the Aztecs did not build Teotihuacan, they discovered it ... 500 years after it had been completely abandoned....The Aztecs couldn't comprehend how such a vast city had just appeared in the middle of nowhere, so they assumed the gods had built it (Teotihuacan literally means "place of the gods").

Here's Wikipedia's entry.