Marsh gas
Marsh gas, swamp gas, and bog gas is a mixture of methane, hydrogen sulfide, and carbon dioxide, produced naturally within some geographical marshes, swamps, and bogs.
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 has occurred at a natural reserve in Spain. Such fires can cause surface subsidence, presenting an unpredictable physical hazard as well as environmental changes or damage to the local environment and the ecosystem it supports.
