Fossil fuels are fuels formed from the remains of ancient organisms. The three main types are coal, petroleum (oil), and natural gas. Coal forms from peat deposits subjected to heat and pressure over millions of years. Oil forms from the remains of ancient marine organisms. Natural gas forms in association with oil deposits or alone. These fossil fuels are important energy sources that are burned to produce electricity, heat homes, and power vehicles and industry. Refineries process crude oil into useful products like gasoline and plastics. Major oil spills like Exxon Valdez and Deepwater Horizon cause environmental damage that can last for decades.