Fossil fuels are a nonrenewable resource formed from the remains of ancient plants and animals that were buried, then subjected to heat and pressure over millions of years, forming coal, oil, and natural gas. The three main types of fossil fuels - coal, oil, and natural gas - are used widely as energy sources, with coal generating over half of U.S. electricity and oil powering vehicles and producing plastic and other petroleum products. Two notable oil spills, Exxon Valdez and Deepwater Horizon, caused environmental damage and economic impacts.