The site for an ancient temple built by the Aztecs at Tenochtitlan, and destroyed subsequently by the invading Spanish armies, who then built a cathedral at the site. The buried temple chambers were discovered by accident in 1978, and the site was soon reclaimed as a heritage site. Now you can actually see the layers of Aztec construction as you walk down the excavated site. A modern museum, the Museo del Templo Mayor, houses many of the archaeological artifacts dug up at the site, and makes for an interesting visit for children and adults alike.
Admission to the museum is 48 pesos, but is free for students, and children under 12.