The early days of espionage were devoid of the high tech glamour that we're accustomed to seeing in spy thrillers. For decades, secret messages have been broadcast across the airwaves for anyone to listen to -- and they have.

Enthusiasts and conspiracy theorists have been following these broadcasts for almost as long as they have existed. With very little information to go on, they have dubbed these broadcasts 'numbers stations'.

But what are they, exactly? Who is sending these mysterious -- and often creepy -- messages, and why can't anyone seem to crack them?