Short wave transmitter: Broadcasts can carry over 1000s of miles
It is still possible to hear so called numbers stations on the airwaves.
The mysterious short-wave stations broadcast a string of apparently random numbers, usually preceded by a well-known folk tune.
It is widely believed that they are run by intelligence agencies sending coded messages to their agents overseas.
The subject has achieved cult status, with many bands using recordings of the stations in their songs.
Simon Mason from Anlaby has written articles and books on the subject. He has also appeared on many radio and television programmes talking about this secretive world.
He said he became interested in the subject as a teenager whilst playing with the family's radiogram in the early 1970s:
Simon Mason: became fascinated by the stations as a teenager.
"Being a curious teenager, I used to tune around seeing what I could find on the short wave bands. You could hear shipping, aircraft, radio amateurs, broadcast stations such as Voice of America and Radio Moscow. And at the same time I started coming across these weird stations that just had spoken numbers."
The stations work by using a system known as a one-time pad. The agent in the field has a small pad printed with a random sequence of numbers.
The spy agency will have a similar pad with the same numbers. The numbers relate to letters, so the sequence read out by the radio station can be decoded into a message.
As the name suggests each pad is used just the once for each broadcast and then disposed of. This changing of the code makes it nearly impossible for a hostile intelligence agency to crack the message.
In the age of the internet and satellite communication it seems strange that intelligence agencies are still using such a low technology system as short wave radio.
However, the system has advantages over other forms of communication as Simon Mason explained:
The fall of the Berlin Wall led to a decline in numbers stations.
"The beauty about number stations is that transmission could be for literally anybody in the world. There is no way of tracing the individual recipient, or their identity or even the country their in. So that's the key to its effectiveness."
Many of the stations were a product of the Cold War. With the fall of the Berlin Wall in 1989 and the independence of many former Soviet Bloc countries, lots of number stations went off air.
It was not just the Warsaw Pact countries who ran the stations. One of the most famous was the Lincolnshire Poacher station, believed to be run by Britain's MI6 from a base in Cyprus.
The station has now stopped broadcasting, although a sister station is still operating in the Far East.
Despite their mysterious origins number stations have achieved cult status. Appearing in the Tom Cruise film Vanilla Sky. Many bands such as Stereolab and the Porcupine Trees have featured samples of the broadcasts in their songs.
For Simon Mason the fascination is down to the low tech nature of the sound:
MI6 HQ: The source of the Lincolnshire Poacher station?
"They're very other worldly. The sound, the atmosphere they generate is unique. You don't hear that anywhere else, especially with crystal clear CDs and MP3 files. These are from another dimension, another era. Because there are all sorts of interference, strange qualities of the sound, it fades in and out.
And of course the voices themselves, most of them aren't real people, they are synthesised voices. So there's quite a strange ambience that these sounds have."