When a scientist studied the clay layer in the late Cretaceous strata in Gubio, Italy, he found that the content of trace element iridium increased 30-160 times than that in other periods  Later, people took samples from many places around the world and came to the same conclusion: the abnormal increase of iridium content in the late Cretaceous strata is indeed universal
Iridium is not only found in celestial bodies outside the earth, but also in the interior of the earth's crust
 The scientists speculated that, in the late Cretaceous strata, a huge asteroid struck the earth, creating dust that blocked out the sun and changed the surface climate, leading to the demise of the dinosaurs