On September 19, 1985, at 7:19 in the morning, an earthquake of magnitude 8.0 with an epicentre off the Pacific coast of Michoacan, some 350 kilometres away, brought down hundreds of buildings in Mexico City. The official death toll was about 10,000, though estimates run from 5,000 to 45,000, and the government never published a number it would defend. About 250,000 people lost their homes.
The geology did most of the damage. Mexico City was built on the bed of Lake Texcoco, which the Aztecs had also built on and which has been drained over centuries. The old lake bed is soft sediment, and it amplifies seismic waves at particular frequencies: the shaking in the city centre lasted two minutes and was several times stronger than on the rock at the edge of the basin. Buildings between about six and fifteen storeys, whose natural resonance matched that of the ground, failed worst. Very tall buildings survived. Very small ones survived.
The collapse was concentrated in the historic centre and in the Tlatelolco housing complex, where the Nuevo Leon building, a 15-storey block of 220 apartments, pancaked and killed several hundred people, many of them children in a school inside it. Hospitals went down too: the Centro Medico and the Juarez hospital lost whole wings, and patients and staff were trapped. Telephone lines failed, and for the first day nobody outside the city knew the scale.
What happened next is why the date is remembered. The government of Miguel de la Madrid was slow, refused foreign aid for several days, and initially said the emergency was under control. Citizens did the work: thousands dug through rubble by hand, organised rescue brigades, set up soup kitchens, and occupied the ruins to demand housing. The phrase that came out of it was that the city saved itself.
The political effects were lasting. The earthquake and the government's handling of it are usually counted as the beginning of the end for the PRI's grip on Mexico City, and the neighbourhood organisations formed in 1985 fed into the opposition that won the capital's first elections in 1997. Building codes were rewritten, and a seismic alert system was built using sensors on the coast. Drills are held every September 19, and in 2017 a magnitude 7.1 quake struck on the same calendar date.