Methane (CH4) is a chemical substance found in a high percentage in natural gas. It is also a common byproduct of digestion in humans and animals. It is also known as "marsh gas" or "swamp gas" because it is produced when vegetation decomposes in water. Methane is lighter than air and is extremely volatile. It tends to dissipate quickly, but a large enough concentration can displace breathable air.
Naturally colorless and odorless, methane is deadly in high concentrations and is one of the "greenhouse gases".
A tragic example of how deadly methane is occurred on July 2, 2007, when a Mennonite farmer climbed into a manure pit to fix an unclogged pipe. Unbeknown to him, methane gas existed in that pit and it almost instantly killed him. A farm hand, also unaware of the existence of the methane gas, rushed in to help the farmer and then immediately died himself. The farmer's wife and two of his daughters rushed in to help; they inhaled the methane gas and also died immediately.
Methane is found in high quantities in the outer regions of the solar system; it is a major component of Jupiter and Saturn, and is found frozen on Pluto. It is also found in the comets and asteroids of the Oort cloud.