On the northern coast of the Yucatan Peninsula, near the town of Chicxulub, Mexico, is a crater about 120 miles (193 kilometers) in diameter. The asteroid that created this crater was about 6 miles (10 kilometers) wide and hit the Earth 65 million years ago. In spite of these immense measurements, the crater is hard to see, even if you're standing on its rim. To get a good map, NASA researchers examined it from space.
Ten years before the 1990 discovery of the Chicxulub crater, physicist Luis Alvarez and geologist Walter Alvarez, a father-son team, proposed a theory about the impact that created it. They noted increased concentrations of the element iridium in 65-million-year-old clay. Iridium is rare on Earth, but it's more common in some objects from space, like meteors and asteroids. According to the Alvarez theory, a massive asteroid had hit the Earth, blanketing the world in iridium. But a shower of particles wasn't the only effect of the collision -- the impact caused fires, climate change and widespread extinctions. At the same time, dinosaurs, which until then had managed to survive for 180 million years, died out. Geophysicist Doug Robertson of the University of Colorado at Boulder theorizes that the impact heated the Earth's atmosphere, causing most big dinosaurs to die within hours [source: Robertson].
This mass extinction definitely happened. Fossil evidence shows that about 70 percent of species living on Earth at the time became extinct [source: NASA]. The massive die-off marks the border between the Cretaceous and Tertiary periods of the Earth's history, which are also known as the Age of Reptiles and the Age of Mammals, respectively. Today, scientists call the extinction the K-T event after the German spellings of "Cretaceous" and "Tertiary."
The K-T event had an enormous effect on life on Earth, but what would have happened if the asteroid had missed? Would it have led to a world where people and dinosaurs would coexist -- or one in which neither could live?