A maser source in which the water (H₂O) molecule is excited to maser action. There are many different H₂O maser lines. The first to be discovered, in 1969, was the powerful line at 22.2 GHz (13.5 mm) in the Kleinmann–Low Nebula in Orion. Other H₂O lines at higher frequencies can now be detected by instruments such as the Atacama Large Millimeter/submillimeter Array, although those from some lower-excitation states coincide with strong absorption lines due to water vapour in the Earth’s atmosphere and can only be observed from space or by airborne observatories. Water masers are found in environments which are warm and dense enough to provide a sufficient concentration of water vapour, with gas temperatures from a few hundred to about 2000 K, such as in star-forming regions, circumstellar envelopes, and comets. They are also found around the nuclei of some active galaxies in the form of megamasers.