Cuicuilco is an important archaeological Mesoamerican Middle and Late Formative (c. 700 BCE – 150 CE) period site located on the southern shore of the Lake Texcoco in the southeastern Valley of Mexico. Today, it is a significant archaeological site that was occupied during the Early Formative until its destruction in the Late Formative. Based on its date of occupation, Cuicuilco may be the oldest city in the Valley of Mexico and was roughly contemporary with, and possibly interacting with the Olmec of the Gulf Coast of lowland Veracruz and Tabasco.
The decline began in the early 1st century BCE, with the increasing rise of Teotihuacán as a center of cultural and religious influence. By the year 400 CE, the Xitle volcano, located in the vicinity of “the Ajusco” (Nahuatl: atl, xochitl, co, “water”, “flower”, “place”; “place of flowers in water”), also known as Sierra del Ajusco - Chichinauhtzin, erupted burying and destroying what still remained Cuicuilco and Copilco (another important ceremonial center). This disaster led to cuicuilca culture dispersion towards Toluca and Teotihuacan, that hosted a large part of the cuicuilcas and incorporated many features of their culture.