Hidden behind five rings of barbed wire in a park just outside Milton Keynes is an unusual manufacturing workshop.
Never in its eighty-five-year history has it opened its doors to the media - until now.
At Her Majesty's Government Communications Center (HMGCC) in Hensloup Park, objects are made that at first glance seem quite ordinary.
But that's far from the whole story - with a history that includes cryptographer Alan Turing, hermetically sealed rooms and comparisons to top-secret gadgets from James Bond films.
The reason for maximum security?
These items are made for British spies and help them cover up their work.
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After the BBC got exclusive access to this massive complex, we had to hand over our phones and security guards followed us every step of the way.
We got a peek - but not much more - at what goes on inside the compound as HMGCC tries to forge new partnerships to keep its edge in the espionage game.
"We've made it really difficult for people to get in touch with us and that, historically, has been a really good way of working," explains CEO George Williamson.
But he says now is the time for a change - even if it seems a bit "unusual".
With anonymous looking buildings, the place looks like an ordinary industrial complex.
Engineers, physicists, chemists, designers, coders and other specialists work on what is described rather vaguely as “a mix of craft and engineering.
In some areas we have to wear an anti-static suit, while in others they show us an astonishing range of machines.
These include some that look like electronic motherboards, laser cutters, and 3D printers (named Darth Vader, Luke, and Leia in homage to Star Wars).
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But what exactly are the creations that emerge from these machines used for?
Part of the problem is that, despite my best efforts, no one will tell me.
This is because the devices that come out at the other end are strictly confidential.
But you can get a picture based on some clues from the past.
HMGCC was created before the outbreak of World War II when spies and diplomats in Europe needed to communicate secretly and securely with Great Britain.
This led to the creation of secret radio systems that could be smuggled in a diplomatic bag.
Some of them were used by officials fleeing Warsaw during the German occupation of Poland in 1939 to report what was happening there.
By the time war broke out, this had evolved into the manufacture of smaller radios given to MI6 agents who parachuted into enemy territory in occupied Europe to send back intelligence.
During the war, Turing lived and worked in Hensloup Park.
Most famous for breaking Nazi codes at nearby Bletchley Park, he worked at HMGCC to develop a device that would enable speech encryption.
The existing system used by wartime leaders Winston Churchill and Franklin D. Roosevelt weighed 50 tons.
Turing's prototype Delayle transferred noise from a record player to speech.
It was portable, ahead of its time and another hint of what is being made here today.
Modern day spies
"I suspect you can trace the time sequence back to what happened here 70 or 80 years ago," Turing's nephew Sir Dermot Turing tells the BBC.
"The need for secure communication hasn't gone away."
In what way is this reflected in the modern world?
Today, undercover agents who operate in the so-called "denied areas" such as Russia and Iran must communicate.
And while HMGCC refuses to comment, other sources say modern day spies rely on things like transmitters that broadcast in secret bursts.
They can be made to look like ordinary objects and send information in a fraction of a second.
I imagine that's exactly what's being done here - but no one wants to confirm for me.
Another item they show me gives a new hint as to what HMGCC is up to.
It's a car radio speaker that dates back to the thirties.
Hidden in the back is a secret transmitter.
Communication is only one part of the job.
But so, it seems, are covert eavesdropping and monitoring devices, although, again, officials remain remarkably tight-lipped when I ask them.
"For most of the past 85 years we have been building secure communications systems that enable people in often difficult, dangerous and remote locations to communicate secretly with the UK," says Williamson.
"In the case of some of our domestic national security agencies, we can help them with some of their investigative work by producing technology that makes it easier for them to do tasks like surveillance."
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One of HMGCC's customers is MI5, who may need to secretly wiretapping suspects at their UK home or follow them in their vehicle.
This may involve hiding the listening device in an everyday object that no one would notice.
What exactly that might be is another thing no one wants to talk about.
It's hard not to say that this all looks like the Kew department from the James Bond movies.
Insiders claim that the comparison isn't quite right - I'm guessing because they don't make things that explode or cars with rocket launchers, but it's hard to be absolutely sure.
In one rubber-floored room, two staff members test electronic devices to make sure no one is accidentally electrocuted.
Elsewhere, the items made here are tested in extreme heat or cold to make sure they can still receive and transmit signals.
One of the weirder places they show me is called the Stargate, a sealed container decorated with little gray foam spikes.
When they lock me in it, it feels a bit like a modern day version of a medieval torture device.
The room has a carousel that rotates the device, with sensors that test what pattern a particular communication device emits.
This could help figure out how likely you are to be detected by a hostile country and also, perhaps, how you can detect devices being used on your territory.
Within some very tight constraints, HMGCC is opening up now because it knows that new technology that could be critical to its missions is being developed in small start-ups and academia.
That technology could come from fields that have nothing to do with national security - but could have purposes that their creators are not even aware of.
Previously, strict security prevented any cooperation, but there is hope that it is now possible.
"The idea is that now we can take our engineers and their great ideas and put them in the same room with people from industry or academia," says Williamson.
"In that magical moment when different ideas start to come together, something really special will emerge."
But how that technology will ultimately be used will most likely remain a secret, just like almost everything else at this location.
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