Roughly five billion. That is the number of Hiroshima-sized bombs it would take to match the energy of a single morning 66 million years ago.

Yes, according to a Physics Today summary of recent impact modelling, “The collision energy was 3 × 1023 J, which is equivalent to about 5 billion Hiroshima-sized atomic bomb explosions.” The figure is a model-derived estimate, and the Hiroshima comparison is illustrative rather than exact but it still does the work of making an unimaginable event somewhat graspable.

The asteroid was about 10 to 15 kilometres in diamater. It struck the northern Yucatán Peninsula and left the Chicxulub crater, a buried scar some 180 kilometres wide. What followed was not a single catastrophic instant but a chain of consequences that played out over hours, months, and years. The story worth telling is less about the strike itself than about how a brief collision became a planetary reset.

The first hours

The immediate physics were brutal. Simulations summarised in the same Physics Today piece indicate that “Within seconds, the impact excavated a hole more than 20 km deep into Earth’s surface and threw bedrock and sediment into the upper atmosphere.” Again, that depth is a simulation result, not a measurement, but it captures the scale of the excavation.

Material did not just fall back where it came from. A 2022 review in Nature Reviews Earth & Environment, led by researchers at Imperial College London, synthesised the modelling this way: “Several thousand gigatonnes of asteroidal and target material were ejected at velocities exceeding 5 kilometres per second, forming a fast-moving cloud that transported dust, soot and sulfate aerosols around the Earth within hours.” A regional event became a global one before the day was out.

The long winter that did the killing

The strike was survivable for a great many organisms. The aftermath was not. The Nature Reviews synthesis frames the central mechanism plainly: “These impact ejecta and soot from global wildfires blocked sunlight and caused global cooling, thus explaining the severity and abruptness of the mass extinction.” That cooling likely explains why the extinction was so sharp, though the relative contributions of dust, soot, and sulfate aerosols remain debated.

How long the dark lasted is genuinely unsettled. The same review notes carefully that “it remains uncertain whether this impact winter lasted for many months or for more than a decade.” That uncertainty matters, because the duration is the difference between a lean season and the collapse of nearly every food web on the planet.

The debate over what drove the cooling is still live. A 2016 study in Scientific Reports proposed that soot ejected from the oil-rich rock at the impact site, spread globally through the stratosphere, drove much of the cooling and darkening, placing soot from the impact site itself, not just wildfires, near the front of the killing sequence. More recently, a 2025 Nature Communications paper by Katerina Rodiouchkina and colleagues revised the sulfur estimate sharply downward, suggesting a less prominent role for sulfur emission and a milder impact winter, with implications for which species pulled through the first years after the strike. It is one study, and it conflicts with other modelling that treats sulfur as the main early driver. The precise recipe of the winter is still being worked out.

Who slipped through

The pattern of survival is the part that tends to surprise people. Size was close to a death sentence. The Scientific Reports analysis notes that only about 12% of land-dwelling forms survived, with large-bodied land animals probably almost entirely wiped out. The contrast with the water is striking: around 90% of species in the freshwater assemblage survived, buffered because they relied on detritus rather than the living plants and plankton that the darkness destroyed much of. 

Finding the crater, decades later

For years the impact was a hypothesis without a smoking gun. The crater was first detected in the late 1970s by petroleum geophysicists Glen Penfield and Antonio Camargo, but its connection to the extinction was only confirmed around 1990 and 1991, after Alan Hildebrand tied iridium-rich boundary deposits to the buried structure beneath the Yucatán. 

The rock record has since been read directly. In 2016, an international drilling project bored into the crater’s peak ring, recovering shocked minerals and impact rocks that confirmed long-modelled details of the collision. As a summary of that work records, the drilling confirmed the peak ring was built from granite ejected within minutes from deep in the Earth. 

What the record shows, followed to the end, is quietly disorienting. The event erased three-quarters of life on Earth.