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Most of the cultivated species of Archaea are extremophilic, that is they live in environments that, to us, seem extreme. They are usually divided into 3 phenotypic groups. The extreme halophiles are the inhabitants of salt and soda lakes, thriving in saturated brines. The methanogens are extremely anaerobic, requiring not just the lack of oxygen but very low oxidation potential for survival. They make methane (swamp gas, natural gas) for a living, most of them just from CO2 and H2. They are common sediment inhabitants, and also live in the GI tracts of animals. Many are thermophilic. The remaining organisms are sulfur-metabilizing thermophiles, organisms that at least have the capacity to oxidize or reduce sulfur for a living and grow at high - sometimes very high - temperatures; many grow near, at, or above the boiling point. Many of these organisms are also acidophilic, growing at pH's down to zero. Phylogenetically, cultivated Archaea fall into 2 major groups; the crenarchaea and euryarchaea.