In 1978, workmen digging on the east side of the Metropolitan Cathedral, next to the Palacio Nacional, unearthed an exquisite Aztec stone of the moon goddess Coyolxauhqui. Major excavations by Mexican archaeologists followed, and they uncovered interior remains of the Pyramid of Huitzilopochtli, also called the Templo Mayor (Great Temple) -- the most important religious structure in the Aztec capital. What you see are the remains of pyramids that were covered by the great pyramid the Spaniards saw upon their arrival in the 16th century.
At the time of the 1521 conquest, the site was the center of religious life for the city of 200,000. No other museum illustrates the variety and splendor of the Aztec Empire the way this one does. All 6,000 pieces came from the relatively small plot of excavated ruins just in front of the museum. Strolling along the walkways built over the site, visitors pass a water-collection conduit constructed during the presidency of Porfirio Díaz (1877-1911), as well as far earlier constructions. Shelters cover the ruins to protect traces of original paint and carving. Note especially the Tzompantli, or Altar of Skulls, a common Aztec and Maya design. Explanatory plaques with building dates are in Spanish.
The Museo del Templo Mayor (Museum of the Great Temple) opened in 1987. To enter it, take the walkway to the large building in the back portion of the site, which contains fabulous artifacts from on-site excavations. Inside the door, a model of Tenochtitlan gives a good idea of the scale of the vast city of the Aztecs. The rooms and exhibits, organized by subject, occupy many levels around a central open space. You'll see some marvelous displays of masks, figurines, tools, jewelry, and other artifacts, including the huge stone wheel of the moon goddess Coyolxauhqui ("she with bells painted upon her face") on the second floor. The goddess ruled the night, the Aztec believed, but died at the dawning of every day, slain and dismembered by her brother, Huitzilopochtli, the sun god.