What Nick said.

Though I'd like to add that it's probably an impact. Not an asteroid impact, however, but a comet - which is loads more effective. It's basically said to have struck the Earth at an incredible speed some 65 million years ago, killing about 70% of the living creatures that ruled the Earth back then (which were mainly the dinosaurs). The other 30% were killed off due to the huge amounts of dust caused by the impact. This dust is thought to have spread all over the sky and blocked out sunlight for weeks, resulting in the death of the rest of the dinosaurs.

If I'm not mistaken, the comet hit an area somewhere in between North America and South America; near Mexico. And the impact still exists today.

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