Numbers stations are shortwave radio transmissions in which a voice reads out groups of digits or letters, usually introduced by a recurring tune, phrase or electronic interval signal. They are widely believed to send encrypted instructions to intelligence agents, who decode them with one-time pads, but governments have almost never acknowledged them. A rare exception came with the Cuban spy ring known as the Wasp Network, arrested in 1998: at the 2001 trial prosecutors described agents copying numbers from the Cuban station nicknamed Atención and decoding them on laptops.
Listeners name stations after their signals. The Lincolnshire Poacher, which played bars of the English folk tune and is thought to have been run by British intelligence, fell silent in 2008. The Russian station known as UVB-76, or the Buzzer, has emitted a repetitive buzzing tone on 4625 kHz since the early 1980s, occasionally interrupted by a spoken message, an unexplained drone heard around the world. Their disembodied voices and tones have long fed the imagination of experimental and electronic musicians.