65 million years ago, half of all the different kinds of animals on Earth died out. Large land creatures suffered the most. All animals larger than a big dog had disappeared completely, including all the dinosaurs!
Something extraordinary must have happened to destroy so much life. Scientists call this event the Cretaceous-Tertiary event (K-T for short). Nobody is sure what the K-T event was, but scientists have some ideas about what might have happened.
Asteroid explanation 
An asteroid is a giant piece of rock moving through space. Very rarely, these asteroids fall on Earth. Some scientists think that an asteroid 10 kilometres wide fell on Earth 65 million years ago. A large asteroid hitting the Earth would cause a huge fire storm, and cause a lot of dust to fly into the air. This dust would block out the sun for several months, or even for several years.
With most sunlight blocked by dust, the ground would get colder, and a long winter would come. To make things worse, the large amount of dust in the air would cause acid rain, rain so poisonous that it can harm or even kill plants.
The combination of the cold, little sun and poisonous rain would kill many plants. Herbivore dinosaurs would not have enough left to eat, and would starve and without herbivores to eat, the carnivores would starve too.
When scientists examine layers of very old rocks from 65 million years ago, they find a lot of one type of rare metal called iridium. Because iridium is so rare on earth, it must have come from somewhere else, like an asteroid. More evidence of this is an enormous 180 kilometre (111 miles) wide crater in Mexico, called the Chicxulub Crater. An asteroid impact could have made such a crater.
A few scientists disagree with this explanation. They say that all this would cause dinosaurs to die much faster than they did. There are similar big craters around the world that affected ancient life.
Other explanations 
Some scientists think that volcano eruptions could have caused the K-T event. These volcanoes would release huge amounts of ash and poisonous gases into the air. Like the dust in the asteroid explanation, this ash would stay in the air for a long time and cause a winter and acid rain.
These gases would also destroy the protective ozone layer in the sky. The ozone layer protects us from the Sun's harmful rays.
Scientists have found out that there have been some huge eruptions. These eruptions could have covered both the states Alaska and Texas in one kilometre of ash and lava!