Which asteroid impact site was identified from oil-company gravity surveys decades after the fact?

Chicxulub, under the Yucatán. Pemex geophysicists mapped a huge circular anomaly in the 1970s while looking for oil, concluded it was volcanic, and filed it. Meanwhile the Alvarez team had found a worldwide layer of iridium at the boundary marking the dinosaurs' extinction and were arguing for an impact with no crater to point at. The two halves were put together in 1990, largely by a journalist connecting people who had not met.

The crater is about 180 kilometres across and buried under a kilometre of limestone, which is why it was not obvious. Drilling into the peak ring in 2016 recovered the sequence of a single terrible day: melt rock, then breccia, then sediment carrying charcoal, laid down in hours as the sea rushed back in.

The rock it hit mattered as much as the size. Yucatán is sulphate-rich, so the impact injected enormous quantities of sulphur aerosols into the stratosphere and produced years of darkness and cold. Hit almost anywhere else on the planet and the outcome may well have been survivable - which makes the extinction of three quarters of species partly a matter of where the thing landed. The Deccan volcanism in India was going on at the same time and is still argued to have contributed.