Biogas arises in the anaerobic degradation process of organic material. Responsible are microorganisms, mainly bacteria, that process existing oganics under exclusion of air and light. In the 1950s, the word "biogas" was shaped for this gas production in technological plants. 

These degradation processes are executed in many places in the nature. Depending on the point of origin, the gas is called differently.  

Natural gas	fossil "biogas"
Firedamp:	exists in mines and is responsible for special explosions
Landfill gas:	developed in landfills
Sewer gas:	developed in sewage treatment plants
Moor gas:	sometimes responsable for the burning air above marshs, moors and seas, the so called "will-o'-the-wisp
Belch gas:	is developed up to 200 l per day in the digestive tract of ruminants, also called digestive gas

 

Digestive gas and muck gas are further definitions for biogas. The table displays the extent of the annual natural biogas production worldwide.  

biogas source	Annual emission [m. t methane/year]
animal digestive tract	100-200
rice fields	280
marshs, moors and anaerobic mud	190-300
of seas and lakes
firedamp	6-22