Euryarchaeota is one of the four phyla of the domain Archaea. Euryarchaeota comprises a physiologically diverse group of Archaea: all known methanogens, extreme halophilic Archaea, hyperthermephiles such as Thermococcus and Pyrococcus, most acidophilic-thermophilic prokaryotes including Picrophilus and the thermophilic-acidophilic cell wall-less Thermoplasma. A large number of Euryarchaeota produce methane (CH4) as an integral part of their metabolism (see methanogens). It is considered a very ancient metabolism due to the unique set of enzymes involved. The extreme halophilic Archaea are a diverse group of Euryarchaea that inhabit highly saline environments.