The main temple of the Aztecs was only part of a much larger sacred center of the great city of Tenochtitlan, which may have contained as many as 78 buildings. This temple was dedicated to the rain god, Tlaloc, and the god of war, Huitzilopochtli. The temple went through several phases of construction, each covering over previous layers in order to make the building bigger.
Excavation of the great temple began in 1978, when the stone sculpture of the moon goddess Coyolxauqui was unearthed by electric company workers. This piece and many others found here are on display in the Templo Mayor museum which was inaugurated in 1987.
You can see the ruins from the sidewalk, or pay the admission fee which allows access to the ruins and the museum. Read more about the Templo Mayor archaeological site and museum.