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New documents recently released by GCHQ at the Public Records Office tell the story of the decryption of Hitler's secret codes and the construction of the world's first ever computer, Colossus.

The paint is peeling on the low wooden huts, their foundations cracked, their eavestroughs broken and mossy. Blue tarpaulins flutter on the roofs, offering scant protection from England's notoriously changeable weather.

It may seem like a ramshackle holiday camp, but this is the place where the famed mathematician and computer pioneer Alan Turing and his code-breaking colleagues laboured to bring an end to the Second World War, and by some estimates shortened it by two years. It's not a derelict housing estate but the birthplace of the modern computer. Bletchley Park, one of the most important historical sites in Britain, is finally struggling back from ruin.

After decades of neglect, there's been a resurgence of interest in Bletchley, with visitor numbers rising to 150,000 this year. For years it received no government funds, but a recent grant of ₤5-million from the British Heritage Lottery Fund and ₤500,000 from Google have brought the park close to its ₤7.5-million renovation goal.

It could have been worse: This estate – where code breakers such as Turing, Mavis Batey and Dilly Knox once worked feverishly to decipher German messages with pencil and paper, and then with early computers – was nearly razed to make way for a supermarket in the early 1990s. For five decades after the war's end, a government policy of hiding its intelligence capabilities, combined with the awe-inspiring ability of its 8,000 wartime employees to keep completely silent about the work they'd done at Bletchley, meant that nobody really knew what happened on this country estate 80 kilometres northwest of London. It was left to crumble, and its memories with it.

"The work done at Bletchley made a huge difference during the war, and had an enormous impact after," says Simon Greenish, director of the Bletchley Park Trust, who is heading the restoration effort. "To lose all that history would be insane."

Last month, Google announced a donation of £500,000 ($780,000), inspired by the rebuilding of Colossus in 2007, the proto-computer used by Bletchley to break Germany's seemingly impregnable Lorenz cipher. "Google recognized that this is the site where the modern world we all inhabit got started," Mr. Greenish says. "The first computer to actually do something was Colossus."

What it did, in part, was make D-Day possible by decoding German messages that showed the Nazis had swallowed the Allies' deception about the invasion landing site. You can see the rebuilt Colossus – giant, clattering, blasting with heat – tucked away in a part of Bletchley that's already been refurbished, and renamed the National Museum of Computing.

But it's the story of almost surreal human effort that continues to fascinate. For six years, young people – they were almost always in their early twenties – worked 24 hours a day in eight-hour shifts to crack the Enigma code used by the German army, navy and air force (as well as encrypted messages from Italy and Japan). They worked in dim, draughty, makeshift huts that weren't in much better shape then than they are now. Crucially, they spoke to no one about their work – not even each other.

Bletchley, once a country estate, lies equidistant between Cambridge and Oxford universities, and many of its code breakers were drawn from the mathematics departments of those two schools. But the intelligence service also drafted chess champions, students of German, women who were good at crosswords and historians.

One of those historians was Asa Briggs, who was recruited from the army signal corps because he'd invented a secret language as a child, and thus showed an aptitude for puzzles. The privations of working at the park, the long hours and uncomfortable lodgings, meant little: "We had a sense of common purpose," says Lord Briggs, who has just written a memoir about his time at Bletchley, called Secret Days. "We were engaged as a team in doing something that was absolutely essential to winning the war."

Some members of the team would become more celebrated than others. Many of those were deeply eccentric, or as one female code breaker put it in Sinclair McKay's book The Secret Life of Bletchley Park, "they were quite mad, some of them, quite potty, but very, very sweet." Angus Wilson, who would become a noted novelist, once leapt fully clothed into Bletchley's pond in a fit of frustration. Mr. Turing, Cambridge's great mathematician, rode his bike around the park wearing a gas mask. In Hut 8, where the cryptographers worked on naval Enigma, there's a replica of his office, complete with a mug chained to a radiator; he didn't like to share his things.

Mr. Turing was broken in the postwar period, hounded to suicide in 1954 after standing trial for homosexual activities. The British government has since apologized to him, and a stamp was recently issued in his honour. This year marks the centenary of his birth in June, and Bletchley plans a special exhibit to commemorate his work.

For the lesser-known code breakers, who are all in their late eighties or nineties, the restored park will be a memorial to their secret toil. This is the most astonishing part of the story: Husbands and wives who worked together at Bletchley never spoke to each other about what they did, even long after they'd left; fathers went to their graves not knowing of their children's war efforts. Only in 1974, with the publication of Frederick Winterbotham's book The Ultra Secret, was the silence finally broken – even then, many code breakers kept mum.

"We were not keeping secrets because we'd signed the Official Secrets Act," Lord Briggs says. "We were keeping secrets because we realized that if our activities at Bletchley were known it would be very deleterious to winning the war, and we all wanted to win the war."

As part of the restoration plan, the surviving code breakers' stories are all being recorded. At a gathering of Bletchley veterans two years ago, Simon Greenish was moved to hear what price they'd paid for their silence: "Some of them said they couldn't get jobs after the war because they had no CVs, and they couldn't tell people what they'd been doing. So they ended up with worse jobs than they should have done, or no jobs at all. But they weren't bitter. They just regarded it as one of life's little trials."

Mr. Greenish hopes that the refurbishment of all the buildings will be finished by 2014, including a new visitor centre to welcome upwards of 250,000 guests each year (compared with 150,000 now). The point, he says, is to preserve "the historical integrity" of what were always quite ramshackle buildings – not to make them any more elaborate, but to stop the rot.

The trust hopes to reach its fundraising goal by Easter, thanks to a grant of ₤5-million from Britain's Heritage Lottery Fund, the first major injection of public money toward the restoration. "The site's now in relatively good order, especially compared to what it was," Mr. Greenish says. "And I think we're fairly solid for the future."