Who or more likely what caused it is unknown and a subject of great debate. Scientists believe that climatic changes might be responsible for most of these extinctions. There is evidence that shows the temperature dropped at the end of the Mesozoic and the sea levels fell. A sudden change in climate might affect the growth of plants and the dinosaur's food supply. One of the most intriguing theories suggested for dinosaur extinction is the asteroid theory.
Iridium is rare on Earth, but abundant in meteorites.
The Alvarezs' suggested that a huge asteroid or comet, perhaps miles in diameter, hit the Earth at that time. The result of such an impact would be an enormous explosion that would throw dust clouds into the sky darkening the planet. Massive forest fires, triggered by the hit, would add smoke to the sky. This would cool the planet causing the climatic changes observed. A crater, now worn down and partly under the ocean, was found along the Mexican Yucatan Peninsula.