The Exxon Valdez oil spill occurred in Prince William Sound, Alaska, on March 24, 1989, when Exxon Valdez, an oil tanker bound for Long Beach, California, struck Prince William Sound's Bligh Reef at 12:04 local time and spilled 11 to 38 million US gallons (260,000 to 900,000 bbl; 42,000 to 144,000 m3) of crude oil over the next few days. It is considered to be one of the most devastating human-caused environmental disasters. The Valdez spill was the largest in US waters until the 2010 Deepwater Horizon oil spill, in terms of volume released. However, Prince William Sound's remote location, accessible only by helicopter, plane, or boat, made government and industry response efforts difficult and severely taxed existing plans for response. The region is a habitat for salmon, sea otters, seals and seabirds. The oil, originally extracted at the Prudhoe Bay oil field, eventually covered 1,300 miles (2,100 km) of coastline, and 11,000 square miles (28,000 km2) of ocean.
According to official reports, the ship was carrying approximately 55 million US gallons (210,000 m3) of oil, of which about 10.1 to 11 million US gallons (240,000 to 260,000 bbl; 38,000 to 42,000 m3) were spilled into the Prince William Sound. A figure of 11 million US gallons (260,000 bbl; 42,000 m3) was a commonly accepted estimate of the spill's volume and has been used by the State of Alaska's Exxon Valdez Oil Spill Trustee Council, the National Oceanic and Atmospheric Administration and environmental groups such as Greenpeace and the Sierra Club. Some groups, such as Defenders of Wildlife, dispute the official estimates, maintaining that the volume of the spill, which was calculated by subtracting the volume of material removed from the vessel's tanks after the spill from the volume of the original cargo, has been underreported. Alternative calculations, based on the assumption that the official reports underestimated how much seawater had been forced into the damaged tanks, placed the total at 25 to 32 million US gallons (600,000 to 760,000 bbl; 95,000 to 121,000 m3). (Full article...)