Natural gas is a natural source of energy, mainly containing methane. This source of energy is used for generating power for machines and vehicles, producing electricity, and providing heat. Natural gas is usually found deep in rock formations or hydrocarbon reservoirs. Natural gas is the cleanest fossil fuel, when compared to coal and oil, as it produces less carbon dioxide. A new method of extracting natural gas through hydraulic fracturing, otherwise known as "fracking," has opened more potential reserves. Fracking has enabled the U.S. to have one of the world’s largest recoverable reserves of natural gas.