Screaming Penguin - Source Does Matter

Scientists have long believed that the impact from a large asteroid could have caused the demise of the dinosaurs. An impact large enough would not kill everything on contact but could theoretically put enough ash and debris in the air to block out the sun for a period of months or years. This lack of light would eliminate the food source for herbivores, and as they died off, so would carnivores, etc.

In the past few decades evidence of a crater large enough to have possibly been from an asteroid large enough to have possibly been the "global killer" has been found on the Yucatan. Thats a lot of theory and several "possibly"s but even more evidence has now been discovered thanks to a high-resolution map from NASA's Shuttle Radar Topography Mission (SRTM).

The "map" has "provided the most telling visible evidence to date of a 112-mile (180-kilometer) wide, 3,000-foot (900-meter) deep impact crater, the result of a collision with a giant comet or asteroid on one of Earth's all-time worst days." The crater is named "Chicxulub".

For more, including the pictures, see the linked National Geographic story.
  Dinosaur-Killer Asteroid Crater Imaged for First Time