Twenty years ago, the oil tanker Exxon Valdez was exiting Alaska's Prince William Sound when it struck a reef in the middle of the night.
What happened next is considered one of the nation's worst environmental disasters: 10.8 million gallons of crude oil spilled into the pristine Alaskan waters, eventually covering 11,000 square miles of ocean.
Now, imagine 8 to 80 times the amount of oil spilled in the Exxon Valdez accident.