Shortwave number stations
I used to hear these, as a child, but never grasped how exciting they were. English, German, Spanish and all sorts of other voices reading numbers, seemingly in some sort of sequence, sitting beneath the static. Others were in Morse code or started with electronic music, similar to a Stylophone or musical doorbell.
Radio 4′s Tracking the Lincolnshire Poacher takes its name from one of the better known number stations. It begins with the folk tune The Lincolnshire Poacher, played on musical doorbell, then continues with a sequence of numbers read in a sampled British woman’s voice. I think it’s a shame that, recently, the station was tracked down to an RAF base in Cyprus; takes some of the mystery out of it. It does, though, give weight to commonly held belief that number stations are government communications to spies in the field.