Fossil fuels are natural resources formed from decayed organic matter over millions of years. There are three main types: coal, oil, and natural gas. Coal is a solid fossil fuel formed from decayed land plants. Oil is a liquid fossil fuel formed from marine microorganisms and is the most widely used. Natural gas is a gaseous fossil fuel also formed from marine life. Refineries process crude oil into useful petroleum products through complex engineering processes. The Exxon Valdez spill in 1989 and Deepwater Horizon spill in 2010 were two of the worst oil spill disasters, damaging wildlife and coastlines.