The extinction of the dinosaurs was caused by a catastrophic event that occurred around 66 million years ago. The most widely accepted theory is that an asteroid, approximately 10 kilometers across, struck just off the coast of the Yucatan peninsula, causing a massive impact that threw huge amounts of debris into the air and caused massive tidal waves to wash over parts of the American continents. The impact also caused substantial fires and dense clouds of dust that blocked the suns rays, darkening and chilling Earth to deadly levels for most plants and animals. The resulting "impact winter" lasted for decades and caused the extinction of not just the dinosaurs, but of up to 70 percent of all plants and animals living at the time.

While the asteroid impact is the most widely accepted theory, there is still some debate over the exact nature of the catastrophic event that caused the extinction of the dinosaurs. Some scientists suggest that volcanic eruptions that caused large-scale climate change may also have been involved, together with more gradual changes to Earths climate that happened over millions of years.

It is assumed by many that the dinosaurs went extinct very quickly following the asteroid strike, but global mass extinction took a while, and it affected certain species more dramatically than others. Many of the small mammals that lived among the dinosaurs, for example, were able to survive because they lived in burrows and could eat just about anything. In addition, species that lived in fresh water typically fared better than those that lived on land. Many researchers now believe that the K-Pg mass extinction event came at a time when the world was in environmental flux and life was already struggling.