CNG stands for Compressed Natural Gas, an alternative fuel source for buses and other vehicles. Though it still produces greenhouse gases, it is more environmentally friendly than gasoline or diesel fuel. It is made bycompressing natural gas, mainly consisting of methane, to less than 1% of its standard pressure. One advantage of using CNG, besides the fact that it's environmentally friendly, is that the fuel systems are sealed, so no leakage occurs. It's also less likely to catch fire than other fuels. However, a lot of space is required for CNG. Most buses store it on the roof.