Naturally occurring methane is generated by anaerobic fermentation, where bacteria break down organic matter producing hydrogen (H2), carbon dioxide (CO2) and methane (CH4). This process naturally occurs in the digestive system of domesticated and wild ruminants, natural wetlands, and rice patties. In ruminants, methane is produced mostly by enteric fermentation where microbes decompose and ferment plant materials, such as celluloses, fiber, starches, and sugars, in their digestive tract or rumen.