I suppose I must have encountered the concept at some point reading spy fiction, but my true introduction to numbers stations came earlier this year from my friend Anthony Atamaniuk. When he played a few examples at a party, I was instantly transported to the nightmarish world of my earliest childhood memories, where the universe beyond my bedcovers seemed to vanish with the fall of night, and every half-heard or half-imagined sound was like a transmission from a cinder planet light-years dead. To me the recordings sounded like outer space, like eternal night, like death.
If you don't know, numbers stations are the shortwave frequencies on which spies regularly transmit coded messages. In 1997, Irdial-Discs released The Conet Project, a 4-CD collection of numbers-station recordings. The whole thing is available as a free download, but you can also purchase a re-release that adds a fifth disc of more recent recordings.
If you have any interest in the history of espionage, or just in very creepy recordings that probably influenced many a horror movie soundtrack, you have to take a listen: