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Going behind the wire at GCHQ (Government Communications Headquarters) in Cheltenham is an odd experience.

It's a place that many people would like to see, but the opportunity is a rare one.

The Oakley site is now being decommissioned with the last few staff members leaving before Christmas.

This will end the final phase of relocation to the nearby Benhall site, nicknamed "the doughnut".

My guide is Tony, GCHQ's historian. Staff are not allowed to divulge their surnames for security reasons.

Of the work that was carried out here since the site opened in 1951, Tony said: "It was mainly classified work. This was the headquarters site and much of the operational work, as well as the administrative work was done on this site."

Walking around the abandoned offices in M Block is eerie. It is strewn with old telephones and unclassified paperwork - plus waste bins marked "classified waste only". If the walls could talk.

Then, at the end of one of top floors are the old directorate offices, and a huge vault door with combination locks.

Inside are rows of filing cabinets and an old secure briefcase - this was where the top brass of GCHQ once locked away their secrets.

Early supercomputers

Through dark corridors with flashing emergency lights and "restricted entry" signs I was taken into one of the computer halls.

Here the early supercomputers built by GCHQ engineers, were installed. I asked Tony how many of these rooms are on site. The only answer I could get from him was "several".

To get a hint of the computing power at Oakley, I was shown the chiller units. Massive pumps are used to cool down all the computer rooms.

When they were fully operational they used 0.5m gallons of water. That is a lot of computing power to keep cool.

Next door are the huge generators which kick in in the event of a power cut. They can generate enough power for the whole of Cheltenham.

Later I meet Keith. He is about to retire after 30 years. He had a job that he has never been able to talk about to anyone outside GCHQ.

He said: "It's something you get used to. I think it's more difficult on the bad days because sometimes you cannot share it. But there's a lot of support in the department."

The reality is that just a few weeks ago, there was work going on at GCHQ Oakley that was highly sensitive and highly classified.

If we had asked to visit then, the answer would have been a resounding no. So why now?

Certainly because there's nothing secret to see anymore, but also there is a desire from GCHQ to be more open.

And with this site destined to become housing, this was a last chance to share the history of a site that not only changed the town, but helped shape the world.