Some of these are included among the methanogens, a group of archea named for the unique way they obtain energy: They use carbon dioxide to oxidize hydrogen, releasing methane as a waste product. Among the strictest of anaerobas, methanogens live in extreme environments, such as kilometers of ice in Greenland, others live in swamps and marshes where other microorganisms have consumed all the oxygen. The "marsh gas" found in such environments is the methane produced by these archaea. Other species of methanogens inhabit the anaerobic environment within the guts of cattle, termites, and other herbivores, playing an essential role in the nutrition of these animals. Methanogens also have an important application as decomposers in sewage treatment facilities.