Ninety percent of the commercial energy (this excludes wood used by households as fuel) consumed today comes from coal, oil, and natural gas. These fuel sources are called fossil fuels because they were made millions of years ago by a combination of dead organic material and geological processes. Most of the coal that exists today was formed 285 to 360 million years ago during the Carboniferous period. At that time there were extensive swamp forests. When the vegetation died, the swamps prevented decay and the organic material built up as peat. Over millions of years, as sediments piled on top, heat and pressure converted peat into coal.