On 21 February 1978 a crew from the electricity company, digging behind the cathedral in the centre of Mexico City, struck a carved stone disc more than three metres across. It showed a woman dismembered: Coyolxauhqui, the moon, cut apart by her brother Huitzilopochtli at the foot of the mountain of Coatepec. The disc lay where the myth said it should lie, at the base of the stair of the Templo Mayor, and it settled the location of the temple that the Spanish had levelled and built over four and a half centuries earlier.
What followed was not a single excavation but a long one. The temple had been enlarged at least seven times, and Mesoamerican practice was to build the new stage over the old rather than to clear it, so each rebuilding sealed the one beneath. The result is a stack of datable envelopes, and buried in them are more than two hundred offering caches: worked stone, sacrificial knives, sea creatures brought from both coasts, jaguars, eagles, and human remains.
The building carried two shrines on one platform. The southern one was for Huitzilopochtli, the tutelary god of the Mexica and the god of the sun and of war. The northern one was for Tlaloc, far older than the Mexica themselves, the god of rain and of the mountains that hold it. Two gods on one summit is a statement about what the city thought it was for. The site museum opened in 1987, and excavation has continued outward into the surrounding streets ever since, under a programme that works around the standing colonial city rather than through it.