An eroded impact crater approximately 150 km in diameter near the city of Mérida in the Yucatán peninsula of Mexico. It was formed 65 million years ago by the impact of an asteroid or comet of estimated diameter 10 km. The environmental effects caused by the ejecta from the impact are thought to have been responsible for the extinction of the dinosaurs. The crater is now buried under 1 km of limestone sediments and was discovered in the 1990s by geophysicists exploring for oil.