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The Â鶹ԼÅÄ's Security Correspondent Gordon Corera gains exclusive access to Britain’s ultra secret listening station where super computers monitor the world's communications traffic. It's also where Britain's global eavesdropping and electronic surveillance operations are conducted.

The layers of secrecy which have surrounded the Government's Communications Headquarters (GCHQ) work are peeled away - but what exactly does it do and who is it listening to?

Gordon 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.

He speaks to a team from the counter-terrorism section who describe what it's like to listen in on terrorists’ conversations - and the constant battle to predict where the next attack will come from.

Code-breakers also 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.

Gordon investigates 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 the GCHQ in Cheltenham, England.

He also explores the scientific and mathematical breakthroughs which have been achieved at GCHQ, including the discovery of public key encryption, which is used when we shop on the internet.

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? Gordon provides the answers.