Buried in the dirt across the globe is a thin layer of dust sprinkled with iridium, a silvery-white metal abundantly found in asteroids. This is the Cretaceous–Paleogene boundary, which is some of the most solid evidence we have that a giant space rock wiped out all of the non-avian dinosaurs 66 million years ago. The impact was so massive and deadly, it killed off 75% of all animals and plants on Earth, leaving behind a scar that persists today.
Such a devastating collision could happen again — and with equally catastrophic extinction. Would humans be prepared if a comet or meteor was suddenly aimed right at us? We can't know until it happens.
