Scientists are still searching for the answer to this longstanding planetary science question. At the moment, it depends on whom you ask.
What we do know is that 66 million years ago, an asteroid smashed into modern-day Mexico near the tiny town of Chicxulub. Land turned to liquid. Twelfth-magnitude earthquakes launched landslides across the Western Hemisphere. And a climate-altering dust blanket enveloped Earth, killing much of its life, including the dinosaurs.
Astronomy magazine subscribers can read the full answer for free. Just make sure you're registered with the website.