Nestled within Mexico City is a little-visited circular ruin. The temple mound is at the heart of what’s believed to be the oldest city in the Valley of Mexico and was built by a civilization that to this day remains a mystery.

Evidence suggests that Cuicuilco began as an early farming settlement around 1200 BC. By 800 BC, its population had expanded. This increase in urban growth and cultural innovation led to the construction of its great pyramid, as well as other small pyramids and dwellings.

Although the culture of Cuicuilco remains a mystery, many of the artifacts discovered in archaeological excavations suggest that the Cuicuilca were a civilization whose religious life centered around the worship of an ancient Mesoamerican deity known as the “Old God of Fire.” In an ironic twist of fate, it was likely fire that brought about the civilization’s abrupt decline.

In the first century AD, the Xitle erupted, sending molten lava spewing down the mountainside and throughout the valley below. Much of the area—including some of its people—were buried beneath lava and suffocating clouds of ash.

Archaeologists believe the survivors of the Cuicuilca civilization fled, abandoning the ruins of Cuicuilco and forming other communities in distant parts of the valley of Mexico. These communities would one day experience a cultural renaissance that gave rise to the great civilization of Teotihuacan, making the Cuicuilca an ancestral culture of Teotihuacan.