One of the common fantasy scenarios are swamps, marshes, bogs, tar pits or even sewers, with a dark and loomy atmosphere, and gas bubbles popping everywhere. What causes these bubbles? Is it the heat, a specific gas coming from the earth or other phenomenon?
Swamp Gas, decomposing plant material in the bog or swamp.
Methane is the primary gas that makes up the product colloquially known as "marsh gas". Much of the biogenic methane produced in nature is derived from either acetate cleavage or by the hydrogen reduction of carbon dioxide. Methane can also be produced by methanogens, archaea that produce methane under anoxic conditions, in a process known as methanogenesis. Methanogenic genera Methanosarcina are common in marsh environments. They are both known to stimulate methane production in aquatic muds and use acetate, methanol, and trimethylamine as substrates for methane production.
Also a bonus google search
