Cuicuilco was an ancient city in the central Mexican highlands, 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 Mesoamerican Middle and Late Formative (ca. 700 B.C. to A.D. 150). 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 (also known as the Olmec heartland). Cuicuilco was originally founded as a farming village, but presents evidence of early religious practices including stone offerings and the use of ceramics as grave goods. The city grew around a large ceremonial center with pyramids and an associated urban area that included plazas and avenues bordering a series of small, shallow pools. These pools were fed by runoff from the nearby hills of Zacayuca and Zacaltepetl. Population at the city's peak is estimated at 20,000 people. Site features include terraces, various buildings, fortifications, and irrigation ditches and canals. Archaeologists conclude that Cuicuilco was a prominent community prior to the emergence of Teotihuacan as an urban center, noting that the six small communities which some archeologists believe eventually combined to become Teotihuacan were founded and showing evidence of modest growth during the time Cuicuilco was building pyramids and public monuments. The city seems to have been abandoned around A.D. 150 to 200 after the eruption of a nearby volcano, Xitle, although the territory was reoccupied at a much later date. Pottery, and other evidence, suggest that refugees from the volcanic disaster migrated north and became part of the population pool of Teotihuacan, near the northern shore of the Lake Texcoco.