When I was a kid, my folks got a multi-band radio that picked up not just AM and FM, but shortwave and audio channels from TV. (If memory serves, they got the radio with a big stack of game tickets from an arcade in Atlantic City.)
While we mostly used it to listen to Johnny Walker or Brian and O'Brien in the mornings, or listen in on TV shows while in the tub in the evening, every once in a while I'd fool with the shortwave. Sometimes I'd pick up a weird channel with someone reading numbers, ghostly voices reciting nonsense.
Years later I learned that these "numbers stations" were coded broadcasts to spies.
The Washington Post recently had an article (originally from the New York Daily News) on these stations. Slashdot discussion led me to the Internet Archive's collection of recordings of them. There's something distinctly creepy about them...might be a good Halloween soundtrack.