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 (see halophile). Most species require 2–4 molar salt concentration for optimal growth and they are able to grow in the presence of saturating salt conditions. Picrophilus is capable of growing below pH 0. Pyrococcus, which literally means fireball, has an optimal growth at 100°C.