Silent is one of the best adjectives for describing the experience of looking at the sky on those especially pellucid nights when the moon and clouds are absent. In the winter most of all, when the night is free of the endless buzzing and chirping of insects, it’s possible to feel how thin the boundary is which lies between you and the true night of interstellar space.
And yet, if you had the ears to hear it, you could directly perceive that the universe is really an incomprehensibly vast instrument. Everything from galaxies to molecules emit a song of electromagnetic radiation which has been bombarding the Earth since long before man learned to listen.
But this noise isn’t evenly distributed. There are relatively quiet regions, such as the ‘microwave window’, which facilitate probing the heavens for signs of artificially-made signals. Near the bottom of the microwave window lies a range of frequencies between hydrogen (H) and hydroxyl ions (OH), respectively vibrating at 1420 MHz and 1660 MHz:
Hydrogen and hydroxyl are two results of the dissociation of water molecules, and are likely audible throughout the universe.
Because of this ubiquity and their position in one of the quietest parts of the radio spectrum, they make an obvious target for any civilization wanting to communicate with other advanced forms of life. And what do we call this meeting place standing between two byproducts of water? The water hole, of course!
It would be fitting, I think, if we were to someday make contact with other sophonts in the same that species have always congregated with their neighbors.