(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.

The pyramid is in the ancient city of Teotihuacan, which is located just northeast of what is now Mexico City ... C.E. By around 500 C.E., it reached a population of 125,000 to 200,000.

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.