Methanogen

Methanogens are archaea that produce methane as a sulfates are depleted, below the top layers.^[2] Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of the earth's crust, kilometers below the surface. There are over 50 described species of methanogens, which are paraphyletic and all included among the Euryarchaeota.