It involves a major earth impact by an asteroid or comet. This catastrophe is thought to have extinguished up to two-thirds of all animal and plant species living at the time. This article explores the dinosaur-collision connection, a continuing scientific debate. They were studying a one centimeter-thick clay layer in Italy which forms a boundary between the Cretaceous and Tertiary geologic periods. The layer is commonly called the K-T boundary, using the German spelling for the word Cretaceous, and marks the assumed extinction of the dinosaurs, about 65 million years ago.
Iridium is known to be more abundant in meteorites, asteroids, and comets than on earth.
Thus the discovered iridium concentration supports the idea of dust fallout from an extraterrestrial impact. Iridium was later found in the K-T layer in locations as widely separated as Italy, Denmark, and New Zealand. The collision idea for the death of the dinosaurs was of limited interest during the 1980s, just one of many proposals. The roughly circular feature is 100-200 miles in diameter. It is buried a kilometer below ground, about half on land and half below the sea floor.