The impact that wiped out the dinosaurs 66 million years ago was caused by a comet or asteroid about 10 kilometers (6 miles) across. The impactor is thought to have struck the Yucatán Peninsula in what is now Mexico. The impact would have caused a massive crater, tsunamis, and wildfires, and would have blocked out the sun for months, leading to a global winter and the extinction of 75% of all plant and animal species on Earth, including the dinosaurs.

The evidence for an impact is based on the discovery of a crater buried beneath the Yucatán Peninsula, known as the Chicxulub crater. The crater is about 200 kilometers (120 miles) in diameter, and it is estimated that the asteroid that created it would have struck with a force equivalent to 100 million megatons of TNT. The impact would have released an enormous amount of heat, dust, and debris into the atmosphere, which would have blocked out the sun and caused global cooling.

In addition to the Chicxulub crater, there is also evidence of the impact in the form of a layer of iridium-rich clay that has been found around the world. Iridium is a rare element that is found in asteroids, and its presence in this clay layer is thought to be the result of the impact.