The waterhole refers to an especially quiet band of the electromagnetic spectrum between 1,420 and 1,666 megahertz, corresponding to wavelengths of 21 and 18 centimeters respectively. The term was coined by American scientist Bernard Oliver. The strongest hydroxyl radical spectral line radiates at 18 centimeters, and hydrogen at 21 centimeters. These two combined form water, and water is currently thought to be essential to extraterrestrial life advanced enough to generate radio signals.
Bernard M. Oliver theorized that the waterhole would be a good, obvious band for communication with extraterrestrial intelligence, hence the pun, in English a watering hole is a vernacular reference to a common place to meet and talk. Several programs involved in the search for extraterrestrial intelligence, including SETI@home, search in the waterhole.
The Daily Omnivore 
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