I’m fascinated by numbers stations, weird broadcasts, usually on shortwave, that seem to relay coded messages between HQ and operatives in the field. It’s all very mysterious and very clandestine. (Read a very good summary of the stations here.)
Here are two examples. First, the Lincolnshire Poacher, which was active from the 1970s until sometime in 2008.
Then there’s the Yosemite Sam station.
These are but two of the world’s most famous numbers stations. Here are eight more. And on a slightly related note, here’s a story on homemade radios in North Korea that are used to illegally up foreign broadcasts.