Marsh gas, swamp gas and bog gas are common names for biogas which forms in wetlands whose principle component of which is methane which is produced naturally within some geographical marshes, swamps, and bogs.
The surface of marches, swamps and bogs is initially porous vegetation that rots to form a crust that prevents oxygen 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 (see: Un insólito incendio subterráneo azota las Tablas de Daimiel). 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.