Natural gas and petroleum are fossil fuels formed from marine life buried in ocean sediments millions of years ago. Natural gas is primarily methane but also contains other alkanes like ethane and propane. It burns cleanly and is used widely for energy. Petroleum contains more complex hydrocarbons and must be refined to separate useful fractions like gasoline. Coal was formed from decaying plants over millions of years and ranges from peat to anthracite based on carbon content and other properties. It releases pollutants when burned but provides an important source of fuel.

1. 25.5 Hydrocarbons from Earth

10. BIG BRUTUS Dragline used to remove the overburden of a strip mining coal field near West Mineral, Kansas Note the man standing beside it