Methane hydrate
Methane hydrate is thought to be formed by dead organic matter trapped between the earth's layers, and then broken down by microorganisms and the earth's heat. The process is similar to the one that produces methane when organic waste is dumped at a garbage pit and covered with earth, except that it occurs deep below the earth's surface. The organisms which produce methane by a process of fermentation are the methaneogen Archea.