Twenty years ago, the single-hulled Exxon Valdez tanker collided with the Bligh Reef in Alaska, spilling 11 million gallons of crude oil into the pristine and ecologically significant Prince William Sound. The massive spill—caused by human error and lack of oversight—ruined one of America's most treasured natural areas and caused the deaths of millions of animals, including more than 3,000 sea otters, 300 harbor seals, 250,000 murres, 14 orcas, and countless fish and benthic invertebrates. Some species are still unrecovered today, and the environment remains blanketed in oil.