Chicxulub impact crater, Yucatán

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A buried impact structure roughly 180 kilometres across, centred near the village of Chicxulub Puerto on the northern coast of the Yucatán Peninsula, half beneath the peninsula and half beneath the Gulf of Mexico, and covered by about a kilometre of younger carbonate sediment. It was first noticed as a circular gravity and magnetic anomaly by geophysicists prospecting for Pemex in the late 1970s and identified as an impact crater of the right age in 1991, more than a decade after the impact hypothesis was proposed — which is why the hypothesis spent the 1980s without a crater. The target rock matters as much as the size: the Yucatán platform is carbonate and sulfate-rich, so the impact vaporised enormous quantities of carbon dioxide and sulfur, and the sulfate aerosols are the leading candidate for the short, severe global cooling and photosynthesis shutdown that followed. In 2016 an IODP–ICDP expedition drilled the crater's peak ring offshore and recovered granitic basement rock uplifted from depth and then collapsed outward, confirming the dynamic collapse model of how large craters form.