The tiered Cuicuilco pyramid is the first large-scale stone monument on the Mexican plateau. It has a base about 370 feet (110 meters) in diameter and currently stands at a height of 60 feet (20 meters), though it was originally much higher.

It is not a true pyramid, being rather a truncated conical mound, with a clay-and-rubble core faced with river boulders and basalt slabs. The summit was reached by ramps on two sides. The circular pyramid is surrounded by smaller structures and rectangular buildings with well-finished floors which may have been houses. Lava from eruptions of the nearby Xitle volcano covers the lower part of the round pyramid.

Radiocarbon dating of finds buried beneath the Xitli lava flows date the initial construction of the "pyramid" to be between 800 to 600 BCE, later remodeling until about 200 BCE, and total abandonment before 150 CE.

A good selections of photographs can be seen here: Cuicuilco Pyramid Archaeological Ruins

The Cuicuilco Pyramid Controversy

Do a search for "Cuicuilco pyramid" on Google and you'll turn up hundreds of pseudoscience/New Age websites claiming that the Xitli lava flows that cover Cuicuilco date to around 6050 BCE, thus making the city itself much older than official dating, and almost as old as the earliest prehistoric dweller found in Mesoamerica.

This dating of the lava deposits was put forth by pseudoarchaeologist Graham Hancock in his book Fingerprints of the Gods, as evidence to support his theory of highly-advanced civilizations existing in prehistory, and seems to have derived from a garbled (and since disproven) estimate made by Byron Cummings prior to the development of radiocarbon dating.

The debunking of this 'revisionist' dating for the pyramid is covered by Paul V. Heinrich of Louisiana State University in his The Wild Side of Geoarchaeology summary.