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Photo: Ralf Roletschek, GFDL 1.2 via Wikimedia Commons
So called because in one city square you can see three different time periods of Mexico City's development mixed together: the pre-hispanic Aztec temple grounds of Tlatelolco, the 16th-century Spanish Church of Santiago, and a modern 20th-century skyscraper, now home of the University Cultural Center Tlatelolco (CCUT) for UNAM.
The Plaza de las Tres Culturas is the main square within the Tlatelolco neighborhood of Mexico City. The name "Three Cultures" is in recognition of the three periods of Mexican history reflected by buildings in the square: pre-Columbian, Spanish colonial, and the independent nation. The square, designed by Mexican architect and urbanist Mario Pani, was completed in 1966.
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Tlatelolco Archaeological Site
The temple, like Tenochtitlan's Templo Mayor, was built in several layers and is now the site of continuing archaeological exploration; it occupies the largest amount of area, on the north and west side of the plaza. If you're not around for the tour, you can guide yourself along the path (complete with English plaques) that takes you through the ruins, which deposits you in front of the Church of Santiago, on the east side of the plaza.
Church of Santiago
The colonial church, built by Spaniards immediately after destroying the temple in their conquest of Aztec lands, was constructed using stones "borrowed" from the temple itself. Despite its weathered appearance, the interior is well-maintained and should still hold regular Mass, although doors may not always be open to the public.
To the south, you'll see the modern-day tower and its adjacent buildings, which were built for the Secretary of External Relations (SRE), now headquartered across the street (though they still have offices in the church's adjoining cloister). UNAM runs the building as a conference hall and cultural center, and has a few exhibits open to the public: Memorial 68 (see Museums, below) and the Blaisten Collection, showcasing modern art.
In UNAM's University Cultural Center Tlatelolco (CCUT) at the Plaza de las Tres Culturas (see Landmarks, above), Memorial 68 is a well-put together museum chronicling the events surrounding the police massacre of student protesters at Tlatelolco in 1968, around the time of the Summer Olympics, which were hosted in Mexico City that year. It's definitely worth a look, but all explanatory text and video interviews are in Spanish only, so brush up your vocabulary or bring a translator.
Megabiblioteca
Next to Buenavista train station, it is the largest library in Mexico and functions as a central library for other libraries in the country. It is housed in an impressive modern building designed by architects Alberto Kalach and Juan Palomar. It is a must for visitors with an interest in architecture, not only for its stunning exterior design, but for its famous "floating stacks" that create the illusion of book shelves hanging from the ceiling. The library also houses a large contemporary art collection. The ground floor houses free temporary exhibits and the grounds include a botanical garden with native plants.
Architecturally unique and utterly fascinating restored neoclassical mansion from the Porfiriato era of the 19th century with period furnishings. Guided tours (in Spanish).