Numbers Stations are mysterious shortwave radio stations thought to broadcast anonymous, one-way communications to intelligence agents. These “Spy Numbers Transmissions” take many forms, but the most common involves the repetitive reading of numbers accompanied by tones or purposeful signal distortion.

The results are as beautiful as they are bizarre. For example, the “Swedish R hapsody” station features a little girl endlessly reading numbers while a distorted music-box melody plays behind her.

The Conet Project has put together a four- disc set of spy numbers recordings. You can also download mp3s of the collection.