Numbers Station: The Gong Station
15 April 2014 Last updated at 18:20 BST
So-called "numbers stations" were mysterious shortwave radio stations that sprang up after World War Two.
Radio listeners started to notice bizarre broadcasts on the airwaves at the height of the Cold War - which some people believe contain secret messages from government agencies.
In this recording, on the station dubbed "The Gong Station", a series of chimes and a woman's voice can be heard.