Coal, oil, and natural gas are composed of the remains of organisms that lived as long ago as 500 million years. These microscopic organisms (such as phytoplankton) became incorporated into the bottom sediments and then were converted, with time, to oil and gas. Coal is the remains of plants and trees (changing into peat and then lignite) that were buried and subjected to pressure, temperature, and chemical processes for millions of years. Fossil fuels are nonrenewable sources of energy. There is a finite supply of the resources for fossil fuels. Eventually these resources will diminish to the point of being too expensive or too environmentally damaging to retrieve. Fossil fuels provide more than 85 percent of all the energy consumed in the United States; including two-thirds of the electricity and nearly all of the transportation fuels.