The waterhole is the radio frequency band between the neutral hydrogen line at 1,420 megahertz (21-centimeter line) and the hydroxyl (OH) line at 1,662 megahertz. It lies in a part of the radio spectrum in which there is relatively little noise from natural celestial sources, so that a directional transmitter of only modest power would be needed to produce a detectable signal over interstellar distances. The waterhole was first identified as a prime region of the radio spectrum in which to carry out searches for intelligent signals in the Project Cyclops report prepared in 1971 by Bernard Oliver and John Billingham:

Nature has provided us with a rather narrow band in this best part of the spectrum that seems especially marked for interstellar contact. ... Standing like the Om and the Um on either side of a gate, these two emissions of the disassociation products of water beckon all water-based life to search for its kind at the age-old meeting place of all species: the waterhole.