Fossil fuels such as coal, oil, and natural gas were formed over hundreds of millions of years from the remains of ancient plants and animals. Coal forms from peat deposits that are compressed and heated over time. Oil forms from algae and plant remains buried under sediment. Natural gas forms in a similar way to oil from the remains of organisms. These fossil fuels are important sources of energy but their extraction and use also causes environmental damage such as pollution and oil spills. The Exxon Valdez oil spill in 1989 devastated wildlife in Alaska's Prince William Sound.