The Mexica told of a migration ended by a sign, an eagle on a cactus growing from a rock in the lake, and built their city on that spot. At its centre stood a single pyramid carrying two shrines side by side. The southern one belonged to Huitzilopochtli, the patron who had led them, born fully armed on the hill of Coatepec and killing his sister Coyolxauhqui as she came to attack their mother; the northern one belonged to Tlaloc, the ancient rain god of the highlands, whose side of the platform was painted blue. One structure therefore carried both the tribal war god and the farmer's rain god, and the two halves are usually read as the dry season and the wet, the mountain of war and the mountain of sustenance.
Rebuilt at least seven times, each stage encasing the last, the pyramid preserved its earlier selves inside it. Excavation since 1978, begun when electricity workers struck a carved disc of the dismembered Coyolxauhqui at the foot of the stair, has recovered more than two hundred offering caches: greenstone masks from other regions and older centuries, corals and sawfish rostra from both coasts, jaguars, eagles, a wolf dressed in gold, sacrificial knives with inlaid faces.
Two later finds have changed the picture again. A monolith of the earth deity Tlaltecuhtli, carved face up and weighing many tonnes, was lifted in 2010, and from 2015 archaeologists have exposed part of the great skull rack described by the Spanish, a tower and platform of human crania that the sources said stood before the temple and that had been assumed to be exaggeration.