Numbers stations are radio stations dedicated to transmitting sequences of seemingly random and meaningless numbers or letters over the air. Not much is known about them, but it is generally agreed that these are encrypted one-way transmissions, designed to deliver a specific, time-sensitive message to one (and only one) person. However, this has never been confirmed or verified, as far as I know. They’ve been around for decades, dating back to the height of the Cold War, and the mind doesn’t have to stretch far to imagine where this might be used.
Usually a synthesized male or female voice gives out numbers repeatedly, though the stations don’t just transmit numbers and letters. Some intersperse music, tones, or singing, and naturally there are stations in various languages, adding to the mystique. If you’ve ever come across any of these over-the-air, I think you’ll agree they can sound spooky and downright creepy. Especially at night.
Here are a few strange ones, via the Internet Archive: