The surface of marshes, swamps and bogs is initially porous vegetation that rots to form a crust that prevents oxygen from reaching the organic material trapped below. That is the condition that allows anaerobic digestion and fermentation of any plant or animal material which incidentally also produces methane.
In some cases there is sufficient heat, fuel and oxygen to allow spontaneous combustion and underground fires to smolder for some considerable time as occurred at a natural reserve in Spain. Such fires can cause surface subsidence presenting an unpredictable physical hazard and as well as environmental changes or damage to the local environment and the ecosystem it supports.
- Landfill gas, produced incidentally where municipal solid waste is abandoned.
- Firedamp, produced naturally in coal mines
- Will-o'-the-wisp, mysterious lights that may be ignited methane
- Flatulence, methane produced within the digestive tract of animals