From the website Big Think comes this video simulation (scroll down) of what would happen if an asteroid hit the earth, as it did over four billion years ago. This asteroid's diameter, 300 miles, is larger than the main island of Japan, and is the same size as the one that hit earth four billion years ago, evaporating the oceans, according to the film.
So what would happen if such an asteroid hit the earth?
First, "The crust of the earth is peeled away like an orange skin." The shattered remains of debris would be hurled out into space. The rim of the crater would be higher than any mountain range on earth today. "And this is just the start," the narrator explains.
Then, the heat. The earth could get as hot as the surface of the sun. "Heat really is the killer," says Brown University's Dr. Peter Schultz. The energy would "literally fry...any living organism that would exist"
One day later, all life on the planet would be vaporized and the earth would look like this:
The oceans would start to bubble and boil. One month later, the oceans would be gone completely. Apparently, an impact like this has happened six times in the earth's history. Let's hope it doesn't happen again.