Some scientists have proposed an atmospheric cause for the extinction of the dinosaurs. In particular, they point to a reduction in the amount of oxygen present in the atmosphere during the Cretaceous period. Researchers recently measured microscopic air bubbles trapped in amber. The study showed that the amount of oxygen present two million years before the end of the Cretaceous period was approximately 35 percent; just after the end of the Cretaceous, the amount was down to 28 percent.