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This is the era of hyper-tech espionage, encrypted emails and mindboggling cryptography. But you can hear a very old-fashioned form of espionage on shortwave radio.

It is 13:03 on a Tuesday in a cramped room with some fairly advanced radio equipment. What is suddenly heard on a shortwave receiving station is a 10-minute message in Morse code.

There is a small community of aficionados who believe messages like this are a throwback to the era of Cold War espionage. They are the mysterious "numbers stations".

At the apex of the Cold War, radio lovers across the globe started to notice bizarre broadcasts on the airwaves. Starting with a weird melody or the sound of several beeps, these transmissions might be followed by the unnerving sound of a strange woman's voice counting in German or the creepy voice of a child reciting letters in English.

Encountering these shortwave radio messages, many radio hams concluded that they were being used to send coded messages across extremely long distances. Coming across one of them was a curious experience. Radio enthusiasts gave them colourful names like the "Nancy Adam Susan", "The Lincolnshire Poacher," "The Swedish Rhapsody" or "The Gong Station."

The Lincolnshire Poacher was so named because of two bars from an English folk song of that name being used as an "interval signal".

Media playback is unsupported on your device Media caption Swedish rhapsody

Times have changed and technology has evolved, but there's evidence that this old-fashioned seeming method of communication might still be used. Shortwave numbers stations might seem low-tech but they probably remain the best option for transmitting information to agents in the field, some espionage experts suggest.

"Nobody has found a more convenient and expedient way of communicating with an agent," says Rupert Allason, an author specialising in espionage issues and writing under the pen name Nigel West.

"Their sole purpose is for intelligence agencies to communicate with their agents in denied areas - a territory where it is difficult to use a consensual form of communications," Allason says.

Media playback is unsupported on your device Media caption Gong Station

A former GCHQ officer, who does not wish to be named, whose duty was to intercept signals towards the UK and search for these numbers stations in the 1980s is also adamant that these were broadcasts to agents in the field or in residencies or directed to embassies.

It was "one-way traffic" - the transmitters broadcast numbers to the recipient. The recipient did not reply.

Why might the numbers stations have been used?

"This system is completely secure because the messages can't be tracked, the recipient could be anywhere," says Akin Fernandez, the creator of the Conet Project - a comprehensive archive of the phenomenon of numbers stations. "It is easy. You just send the spies to a country and get them to buy a radio. They know where to tune and when," he says.

Fernandez was fascinated by the mystery of numbers stations.

Image copyright Akin Fernandez Image caption Any kind of antenna, innocent or otherwise, might once have attracted raised eyebrows

"It was so weird I wanted to know more about them," he says. He put three years of his life aside in order to put together a coherent archive of these stations.

"Once you hear them, it has an effect on you," he says. "I never expected to be talking about it 17 years after hearing it for the first time - when the Conet Project first started."

Unlike other aspects of the Cold War era, the numbers stations didn't leave a lasting impression on popular culture. "It is a dry subject until you listen to them," Fernandez says.

"It is a way of communicating securely between the Secret Intelligent Service and agents, and it is incomprehensible," says Philip Davies, a politics and history professor at the Brunel University in London.

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But espionage was not the only explanation posited. Some people have even argued that the phenomenon was an elaborate prank. But the scale of the stations - multiple frequencies in different languages - makes that explanation seem far-fetched. Fernandez notes that any prankster would need to buy millions of pounds of radio transmitters.

Despite the general veil of secrecy around espionage, the odd bit of corroborative evidence for the purpose of numbers stations has leaked out. "The purpose of numbers stations has been guessed at first by anonymous leaks, stories of people being arrested with radios and 'one-time pads' and other scattered pieces of evidence, as well as some privately published books and magazines," says Fernandez. The one-time pads enabled a form of code that would have been uncrackable to anyone listening in.

In 1989, a Czech spy was arrested in the UK because his equipment was faulty and it radiated into other people's flats. He was unlucky. "When the Ceausescu regime collapsed, there was a cessation of broadcasts from Romania," the former GCHQ officer says.

Experts are confident that numbers stations do still exist, even if there are fewer of them.

"In the same way spy tricks such as pretending to feed ducks around a pond might still exist, numbers stations still exist too," says Al Bolton, a radio amateur. "It is an old-fashioned means of communication but you have to think of security."

Computers almost always leave traces, whereas a paper and a pen are easy to destroy.

Image copyright Getty Images

"The danger with a computer is that if you get caught, the data on it is still retrievable. Whereas with a one-time pad, you can eat it or flush it down the toilet," he says.

In the 2010 raids on a Russian spy ring in the US, court papers alleged that they had used "coded radio transmissions and encrypted data", a hint that they might have received their orders via shortwave numbers stations.

Despite all the clues, no government has ever officially admitted or denied using numbers stations, nor have intelligence agencies.

"Once The Conet Project was released, some spy agencies admitted that they were, 'not for public consumption'. This is as near to an admission that we have been able to obtain," Fernandez says.

Enthusiasts might be fighting sceptics about the stations' real purpose, but what is certain is that they aren't a pure product of imagination.

If you don't believe so "you could always get yourself a short wave radio, wait till the night time and then start scanning for them", Fernandez says.

And then listen and wonder.

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