Templo Mayor (The Great Temple), Mexico City (must see)

A massive pyramid temple in the heart of the Tenochtitlán empire, the Great Temple (Templo Mayor) was once the crown jewel and the absolute center of the Mexica world before the Spanish conquest. First built around 1325, the structure stood almost 60 meters high and had dual staircases leading up to the two crowning temples, one dedicated to Huitzilopochtli, the god of sun and war, and the other to Tlaloc, the god of rain and agriculture.

After the Spanish conquered the city, the Temple was razed, but some of its traces remained. On February 21, 1978, electric company workers digging near the Metropolitan Cathedral uncovered an eight-ton monolith adorned with the carvings of the moon goddess Coyolxauhqui. The discovery prompted a major excavation during which four city blocks were demolished and the base of the Temple was revealed, along with multiple artifacts. In 2017, the space was further expanded to unearth partial remains of a round temple dedicated to the god Ehécatl, which is now open to the public.

The Aztec ruin of such magnitude is rather unique and includes traces of seven pyramids, as well as the Eagle Warrior Temple.

The adjoining eight-room museum showcases an extensive collection of pre-Colombian artifacts found on the site and is well worth two-four hours of your time, depending on your level of interest in Aztec religion, culture, and symbolism.

Tip:
The outside part can get very hot. Make sure to wear a hat and stay hydrated, or avoid the midday sun. If wishing to climb the Pyramid of the Sun, head there first because long lines can form early.
If you are not a Spanish speaker, check if there are audio guides available, because aside from some signage, most of the museum's text content is not translated into other languages.
Visitors can make reservations for a guided tour in English.