Tubular Bells and VLF interference
Remember that album from Mike Oldfield? A user on YouTube found out that this album contains a hidden and unintentional CW message.
This was caused by a powerful VLF station located next to the recording studio, which interfered with the recording equipment. Because the signal is very weak nobody ever noticed it — until now.
The video below shows you how to receive VLF signals with nothing else than an antenna plugged into the microphone input of your sound card and SDRSharp to make it visible and audible.
The decoding of the the Tubular Bells signal is shown at 9:54.