The Soviet radio found in a field near Aberystwyth
A Soviet spy may have been working in the Aberystwyth area during the Cold War.
The revelation comes in the wake of the BBC gaining exclusive access to one of Britain's most secretive buildings.
BBC security correspondent Gordon Corera interviewed staff at the Government Communications Headquarters (GCHQ).
His programme reveals that a Soviet radio transmitter was found near Aberystwyth in the 1960s.
The programme, GCHQ Cracking the Code, can be heard on
until Tuesday, 6 April and reveals that dotted around the GCHQ building are glass cases with exhibits from GCHQ's history.
"This is a simple but robust radio transmitter discovered early in the 1960s in a field near Aberystwyth by a farmer who was ploughing his land," said GCHQ's historian, referred to as Tony during the programme.
"It had obviously been cached there by someone who was working for the Soviets.
"Nobody has a clue who this belonged to, who it was serving, or even which bit of the GRU he was working for."
The GRU is Russia's foreign intelligence network and handled all the collection of intelligence of military or political significance from sources outside the Soviet Union.
The programme compares GCHQ's work to TV series such as Spooks
The programme also explores the wide area covered by signals intelligence - from looking for terrorists planning attacks against the United Kingdom to supporting military operations of the type underway in Afghanistan.
A team from the counter terrorism section describes what it is like to listen in on terrorists' conversations and the constant battle to predict where the next attack will come from.
She said: "I don't think you would be human if you didn't go home at night and couldn't switch off and thought 'Oh my God, What happens if . . .?'"
What about the ethics of eavesdropping and how does their work compare to the way it is portrayed on television in series like 'Spooks'?
Codebreakers talk about their work, attempting to find a chink in the armour of a carefully encrypted message sent by a terrorist or a foreign government.
The building's computer halls covers ten thousand square metres
"It just feels amazing really," when there is a breakthrough, says one. "I mean you feel like you've won".
The programme looks at the technological challenges posed by the internet and the threat of cyber warfare, which has led to the establishment of a new cyber operations centre at Cheltenham.
It also explores the scientific and mathematical breakthroughs which have been achieved at GCHQ, including the discovery of public key encryption, used when we shop on the internet.
There's a tour of the building's four great computer halls, containing racks and racks of IT equipment and covering around ten thousand square metres. "I could actually fit Wembley football pitch into three of the halls quite comfortably,' says the man in charge of making sure that the equipment doesn't crash.
Gordon Corera challenges the director Iain Lobban. There has been considerable speculation about whether the government is planning huge databases at GCHQ to keep track of all communications and internet traffic. Do they really spy on us? And how accountable are they? This programme provides the answers.