When I was writing my book, Jamming the Media (1997), while working on a chapter about pirate radio, I went deep down the rabbit hole of hunting down pirate stations, shortwave stations, police and military radio, and number stations. The latter are mysterious radio broadcasts of unknown origin where a series of numbers are read over the air. These are widely believed to be coded messages to spies in the field from the world's intelligence agencies.
I was surprised to run into this video on Curiosity Droid and to discover that number stations are still a thing. I would have thought that, in the age of encrypted digital communications, number stations would be archaic and unnecessary, but maybe entrenched government infrastructures are hard to dismantle, even when they're obsolete?