The largest natural source of methane, which emits the gas from microorganisms that decompose organic materials in the absence of oxygen, is

Wetlands are the largest natural source of methane, releasing it as microbes decompose organic matter without oxygen.

Enteric digestion in livestock emits methane too but is smaller than natural wetlands.

Fuel gases are human-made or industrial sources, not the largest natural one.

Permafrost stores methane and can release it, yet ranks below wetlands as a source.

Waterlogged, oxygen-poor soils favour the methane-producing bacteria.

Rice paddies act like artificial wetlands and add to methane emissions.