The Water Hole
Two of the most important RF signals from the interstellar medium are the 21 cm line of neutral
hydrogen (H) and the 18 cm line of hydroxyl (OH). Since H + OH forms water, this range of the RF spectrum
from 18 to
21 cm has been termed the water hole. Alien intelligence for which water is critical for
life and with radio telescopes might be expected to choose this region as a logical place to broadcast signals
intended for other intelligent beings to detect.