Morse Code and ELF
The discussion of Morse code reminded me of my first real tech job after I got my BSEE and while I was working on my MS in Computer Science. I was working on a project to understand the propagation of Extreme Low Frequency (ELF) radio waves which were in the range of 15-30 KHz. Since ELF signals propagated around the world and penetrated water a few feet, they were used to communicate with slightly submerged submarines trailing a mile-long antenna. Due the low carrier frequency the signal bandwidth was very small and was used for low-speed Morse communications. Several of my colleagues on the project knew Morse and could transcribe it, but since it was all encoded strings of numbers, I never bothered to learn it.