Number Stations are shortwave radio broadcasts which usually have a repeating sequence of numbers or letters or a mix of both. These recordings can be a little bit psychologically creepy for some reason, perhaps because of the mystery they pose.

Nobody really knows (or at least has told) why these stations exist. It has been speculated that these stations are used to communicate with spies using one time pads.

There has been little luck in finding the transmitters via triangulation

Anyway, these are very cool and I find that even people who are not interested in cryptography are interested in them.