Archaebacteria are scientifically classified within the domain Archaea. The term "archaebacteria" came into use in the late 1970s to describe a newly recognized third domain of life consisting of certain single-celled microorganisms. Since these organisms are wholly different from bacteria, scientists later renamed this domain Archaea.
Many of the Archean microorganisms inhabit harsh environments. Thermophiles live in extreme heat, such as the hot springs of Yellowstone National Park, one of the first places where Archaea were discovered. Halophiles thrive in highly saline waters, while methanogens survive in spite of the methane they produce.
Archaea do not exclusively live under extreme conditions. They can be found in the open sea among plankton.