Great Extinction
A very bad day for dinosaurs. A surprisingly good day for small, burrowing mammals.
The impact
On an otherwise unremarkable day 66 million years ago, a rock approximately 10 kilometres wide arrived from space travelling at 72,000 kilometres per hour. It struck what is now the Yucatán Peninsula in Mexico. The energy released was equivalent to one billion nuclear bombs detonating simultaneously.
What happened next
The impact vaporised rock, triggered a global earthquake, generated a tsunami hundreds of metres tall, and ignited wildfires across entire continents. But none of that was the main killer. The asteroid hit a particularly sulphur-rich geological formation, ejecting billions of tonnes of sulphur dioxide into the upper atmosphere. This blocked sunlight for years. Plants died. Food chains collapsed. 75% of all species on Earth went extinct.
The survivors
The species that survived were small, could burrow underground, could eat insects or roots rather than plants, and could tolerate cold and darkness. Among the survivors were small, rat-like mammals who had been living in the shadow of dinosaurs for 100 million years. With the dinosaurs gone, they rapidly diversified and evolved. One lineage eventually became primates. One eventually became you.
The crater
The Chicxulub crater is 180 kilometres wide and still partially buried beneath the Gulf of Mexico and Yucatán Peninsula. It was only confirmed as the impact site in the 1990s. The crater is so large that when it formed, the Gulf of Mexico briefly had a mountain taller than Everest at its centre — formed by the rebounding crater floor — before collapsing again within minutes.
The extinction event is formally called the K-Pg boundary (Cretaceous-Paleogene). A thin layer of iridium-rich clay found worldwide at exactly this geological level was the first evidence of an extraterrestrial impact — iridium is rare on Earth but common in asteroids. Geologist Walter Alvarez discovered this in 1980. His father, physicist Luis Alvarez, helped him develop the impact hypothesis. It was controversial for a decade before the Chicxulub crater was identified.