Mysteries surround the archaeological complex northeast of Mexico City whose pyramids—the Pyramid of the Moon and the even larger Pyramid of the Sun—remain popular tourist attractions.

(Getty Images)| In Mexico's ancient city of Teotihuacán – a UNESCO World Heritage Site that once boasted a population of more than 125,000 people – visitors will find the Pyramid of the Sun.

Archaeologists have discovered the ruins of an 800-year-old Aztec pyramid in the heart of Mexico City that could show the ancient city is at least a century older than previously thought.