Synthetic fuels, commonly called synfuels, are gaseous and liquid fuels produced synthetically from coal and oil shale. Basically, coal is converted to gaseous or liquid forms in coal-based synfuels. These are easier to transport and burn more cleanly than coal itself. Synthetic natural gas is also produced from coal. Disadvantages of these processes are that they require a large amount of water and the new fuels have 30–40 percent less fuel content than pure coal. Synfuels from oil shale are produced by extracting the oils from the rocky base. Gasoline and kerosene can be produced from oil shale. Synfuel may also be obtained from biomass from human and animal waste. The waste is converted to methane by the action of anaerobic bacteria in a digester.