Teotihuacan is the largest Pre-Columbian archeological site in the Americas.
Teotihuacan is located in San Juan Teotihuacán municipality, in the State of México, Mexico, approximately 40 km (about 25 miles) northeast of the Mexico City, which covers a total surface area of 82.66 km² and, in the year 2000 census, reported a population of 44,653.
Archaeological site
The Pyramid of the Moon
Teotihuacan is a Nahuatl name, meaning Place where there is god-becoming. According to legend this was where the Gods gathered to plan the creation of man.
The name Teotihuacan is also used to refer to the civilization this city dominated, which at its greatest extent included most of Mesoamerica. It is not known who these people were, but there is archaelogical evidence of having been a multi-ethnic place, with distinct Zapotec, Mixtec, Maya and what seems to be Nahua quarters, for example. The Totonacs have always maintained that they were the ones who built it, a story that was corroborated later by the Aztecs. The city was also anciently referred to as Tollan, a name also used centuries later for the Toltec capital of Tula (Tollan Xicocotitlan in Nahuatl).
Construction of Teotihuacan commenced around 300 BC, with the Pyramid of the Sun built by 150 BC. The city reached its zenith approx. 150–450 AD, when it was the centre of an influential culture. At its height the city covered over 30km² (over 20 square miles), and probably housed a population of over 150,000 people, possibly as many as 200,000. Teotihuacan was an important source of obsidian and there was intensive trade with other regions of Mesoamerica.
View of Avenue of the Dead from Pyramid of the Moon
The city's broad central avenue, called "Avenue of the Dead" (a translation from its Nahuatl name Micaohtli), is still flanked by impressive ceremonial architecture, including the immense Pyramid of the Sun and Pyramid of the Moon, the Temple of Quetzalcoatl, and many lesser temples and palaces.
Unfortunately no ancient Teotihuacano texts are known to exist, but the city is occasionally referred to in the texts of Maya monuments, showing that Teotihuacan nobility traveled to and married with the families of local rulers as far away as Honduras. Maya hieroglyphs mention Spearthrower Owl, apparently Emperor of Teotihuacan, who reigned for over 60 years and imposed his relatives as kings of Tikal and Uaxactun in Guatemala.
The city was sacked and burned, possibly by the invading Toltecs, sometime during the 7th or 8th centuries.
Knowledge of the ruins of this huge city was never lost. The ruined city was a place of pilgrimage in Aztec times; it astonished visiting conquistadores; and it has been one of the most noted attractions for visitors to Mexico since the 19th century. Minor archeological excavations were conducted in the 19th century, and in 1905 major projects of excavation and restoration began under archeologist Leopoldo Batres, with the Pyramid of the Sun restored to celebrate the centennial of Mexican Independence in 1910. Teotihuacan remains a major tourist destination; it now contains a museum, and archeological excavations at the site are ongoing.
Pyramid of the Sun 
Info retrieved from wikipedia.org