@Alfred discussing numbers stations got me thinking about The Conet Project.
https://youtube.com/playlist?list=PLSoYU4Rz9Q6xFJ6nPc2zGUPwS_LBzOmUn

@chuck34108 The Conet Project is a four-CD set released in 1997 by the British independent record label Irdial-Discs that catalogs and documents numbers stations. It was one of the first attempts to make these strange and mysterious broadcasts audible to the general public. The project's goal was to study the station's signals and produce a reference book to assist future research. The collection includes over 100 tracks featuring coded transmissions from various numbers stations, as well as some recordings of their corresponding hardware. The recordings, which include transmissions in multiple languages, range from simple Morse code messages to more complex digital bursts. Many of the recordings date back to the Cold War era, when numbers stations were more common. The Conet Project remains a valuable source of information