Fossil fuels such as coal, oil, and natural gas are formed from the remains of ancient organisms over millions of years. Coal forms from peat and contains carbon deposits that provide energy. The main types of coal are anthracite, bituminous, and lignite. Oil forms from the remains of marine organisms and is refined into petroleum products like gasoline. Natural gas forms near oil deposits and is composed primarily of methane. These fossil fuels are non-renewable and are important sources of energy but also cause environmental issues.