When organisms become extinct, they are no longer alive on Earth. Millions of years ago, there was a(n) mass extinction of dinosaurs. Some scientists believe the mass extinction of dinosaurs occurred after a giant asteroid or comet hit Earth. Dust blocked sunlight, therefore plants couldn’t survive. Without sunlight, plants could not make food to survive. Without plants, dinosaurs that ate plants died. Meat-eating dinosaurs also died. Other scientists think volcanoes erupted. This would have created great dust clouds that blocked sunlight. Perhaps a combination of these events caused the dinosaurs’ extinction.