Aliens, ancestors or both?  The Teotihuacan Pyramids predate the Aztecs.  Access the site from this location.

This well-known pyramid in the ancient city of Teotihuacan is one of the largest in the world. 

The Pyramid of the Moon is the second largest pyramid in Teotihuacan, Mexico after the Pyramid of the Sun. It is located in the western part of Teotihuacan and mimics the contours of the mountain Cerro Gordo, just north of the site. Some have called it Tenan, which in Nahuatl, means "mother or protective stone." The Pyramid of the Moon covers a structure older than the Pyramid of the Sun which existed prior to 200 AD. - Wikipedia