For your eyes only: Why is the greatest archive of British cinema stashed away in a top-secret bunker worthy of a Bond villain?

Clearly, they've been watching too many Bond films at the BFI. I'm standing outside a spooky barbed-wire compound in the middle of Nowhereshire, nine miles off the M40. I can't tell you where, because it's a secret, except to say it's a remarkably dreary part of Warwickshire. This place doesn't even have a postcode, as it used to be an MoD bunker, where nuclear warheads were built and stored. Welcome to the home of British cinema, the British Film Institute's Master Film Store.

In a minute, I'll be shown 400,000 canisters of irreplaceable film history, containing everything from Queen Victoria's funeral to Alfred Hitchcock's nine surviving silent films. As of last summer, they have been brought together under one roof – and not just any roof: pass security and into a clearing and there stands a swanky architect-designed grass slope, swooshing up off a £12m labyrinth of airlocks and sealed-off concrete chambers. As I said, think Blofeld.

But why does it all have to be so sinister? The BFI got a bit twitchy when I asked to have a look round, even though we have paid for it, through a £22.5m grant from the Department for Culture, Media and Sport. The public is not allowed in, and never will be, and I am only allowed to visit if accompanied at all times by a dedicated archive press officer, who turns out to be a minor British institution in his own right. Brian Robinson has been at the BFI for 25 years, and is a walking encyclopaedia on silent film.

So come on, Brian, what's the big secret? And why lavish £12m on a stunning piece of architecture nobody can see? The answer to the first question is simple, as anyone who has seen The Artist will appreciate. We all know that Michel Hazanavicius's masterpiece, expected to clear up at tonight's Oscars, is about the moment when silent film was replaced by the "talkies". But only film buffs will have appreciated the significance of the key scene – look away now if you haven't seen it – when our hero sets fire to his life's work.

That's because old film was made on cellulose nitrate, a super-flammable material that degrades really badly. Until the early 1950s, when acetate was introduced, all movies were made on nitrate, which is obviously a nightmare for archivists. This stuff is so unstable that it can spontaneously combust, as it once did in 1970 at the BFI Southbank, seconds after a Fred Astaire number had been loaded. Within seconds, the projection box was destroyed, but the fire didn't spread to the auditorium, thanks to stringent building requirements for cinemas licensed to show nitrate.

The Southbank is now the only cinema in the country licensed to show nitrate films. It's so restricted because once nitrate explodes, there's not much you can do to put it out: it releases oxygen as it burns, fuelling its own flames, and is nearly impossible to extinguish, even under water.

Which helps explain the secrecy: the BFI doesn't want anyone anywhere near this place, because the stock's so damn flammable. One stray fag end and more than 100 years of British culture could go woomph. It would be a national disaster, not to mention a minor embarrassment. The BFI has actually owned this site since the late 1970s, when the MoD decided they no longer needed to store their weapons here. They knocked up a series of Nissen huts which, with the concrete bunkers, were a suitably secret and cool place to store it all. But as anyone who has ever put a Kodak roll in the fridge will know, the best way to store film is somewhere really cold. Scientists have worked out that the optimum temperature at which old nitrate should be stored is -5C, and the new building is essentially a giant freezer. We're kitted out with gloves and fleeces before we go in, and have to go through an air lock to stop the, er, "warm" February air outside coming in.

Four people work at the unit, and they're each wired up to a walkie-talkie system so that they know where everyone is at all times. They all clock on together, and don't start work until they're all on site. Just to add to the spookiness, mobile phones get no reception.

Working in an air-locked freezer is no joke: you're not supposed to stay in there longer than 40 minutes before coming out to warm up. Inside, the building is divided by two long corridors, with discrete concrete chambers on both sides. Each room is filled with floor-to-ceiling mobile stacks of metal canisters, each of which is labelled and barcoded. The barcodes speed up the sorting system, but I'm glad they still have the titles, too: so there's Kind Hearts and Coronets, elsewhere is Powell and Pressburger's The Red Shoes, but it's not just feature films: they've got the original negatives recording Captain Scott's disastrous expedition to the South Pole, and footage of the 1895 Derby. There's something strangely moving about standing here, surrounded by images of the past. It's so grim, sterile and cold, and the fridge mechanism makes a terrible whirring noise, but it's as close as you can get to touching the past.

Which is what, Brian explains, is the point of the £22.5m grant to the BFI. They are not simply preserving these master copies as a monument to the past. All film degrades with use – just think of your old videotapes. So what the BFI has to do is painstakingly clean and copy every reel, frame by frame, in its Berkhamsted laboratories, and put them into formats the public can see. A major project, lasting two years, has been the restoration of all nine of Hitchcock's surviving silent films, in time for this summer's cultural Olympiad. "It's like copying your old family photos," says Brian. "You want your loved ones to look as good as possible. Well, we love the film."

Apart from being cold and shiny, the BFI store is designed so that if one compartment blows up, the fire shoots sideways out of a special flap and meets an earth bank. Each compartment is sealed off by thick concrete dividers. When it was unveiled last summer, a busload of architecture critics was shown round. They loved it. So to my second question: why make it so fab if nobody can see it? As one critic said, they did it to "express the cultural importance of what it contains". Maybe. But I still think somebody's been watching too many Bond films.