Teotihuacan, Citadel, Temple of the Feathered Serpent

Teotihuacan was an ancient Mesoamerican city located in a sub valley of the Valley of Mexico, located in the State of Mexico near modern-day Mexico City, known today as the site of many of the most architecturally significant Mesoamerican pyramids built in the pre-Columbian Americas. Apart from the pyramids, Teotihuacan is also anthropologically significant for its complex, multi-family residential compounds; the Avenue of the Dead; and the small portion of its vibrant murals that have been exceptionally well-preserved. Additionally, Teotihuacan exported fine obsidian tools that garnered high prestige and widespread utilization throughout Mesoamerica.

The city is thought to have been established around 100 BC, with major monuments continuously under construction until about AD 250. The city may have lasted until sometime between the 7th and 8th centuries AD, but its major monuments were sacked and systematically burned around 550 AD. At its zenith, perhaps in the first half of the 1st millennium AD, Teotihuacan was the largest city in the pre-Columbian Americas, with a population estimated at 125,000 or more, making it at least the sixth largest city in the world during its epoch. Teotihuacan was even home to multi-floor apartment compounds built to accommodate this large population. The term Teotihuacan (or Teotihuacano) is also used for the whole civilization and cultural complex associated with the site.

Although it is a subject of debate whether Teotihuacan was the center of a state empire, its influence throughout Mesoamerica is well documented; evidence of Teotihuacano presence can be seen at numerous sites in Veracruz and the Maya region. The later Aztecs saw these magnificent ruins and claimed a common ancestry with the teotihuacanos, modifying and adopting aspects of their culture. The ethnicity of the inhabitants of Teotihuacan is also a subject of debate. Possible candidates are the Nahua, Otomi, or Totonac ethnic groups. Scholars have also suggested that Teotihuacan was a multi-ethnic state.

The site covers a total surface area of 83 square kilometres and was designated a UNESCO World Heritage Site in 1987. It is the most visited archaeological site in Mexico.

The Temple of the Feathered Serpent is the modern-day name for Teotihuacan's third largest pyramid. This structure is notable partly due to the discovery in the 1980s of more than a hundred possibly-sacrificial victims found buried beneath the structure. The burials, like the structure, are dated to some time between 150 and 200 CE. The pyramid takes its name from representations of the Mesoamerican "feathered serpent" deity which covered its sides. These are some of the earliest-known representations of the feathered serpent, often identified with the much-later Aztec god Quetzalcoatl. The structure is also known as the Temple of Quetzalcoatl, and the Feathered Serpent Pyramid.

The Temple of the Feathered Serpent is a six-level step pyramid built in the talud-tablero style. The outside edges of each level are decorated with feathered serpent heads alternating with those of another snake-like creature, often identified as Tlaloc. Under each row of heads are bas-reliefs of the full feathered serpent, in profile, also associated with water symbols. These and other designs and architectural elements are more than merely decorative, suggesting "strong ideological significance", although there is no consensus just what that significance is. Some interpret the pyramid's iconography as cosmological in scope – a myth of the origin of time or of creation – or as calendrical in nature. Others find symbols of rulership, or war and the military.