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Thousands pass it each day: an anonymous black security door on an alleyway leading off Chancery Lane in London. Just ten years ago, the world behind this door was protected by the highest level of state secrecy, as was the disappearance of hundreds of men and women through it each day, down to a complex 60 metres beneath the pavement ready and waiting for nuclear war.

In the UK, the ruins of World War III are all around us: bricked-up doorways across the tube network, tower blocks like Pear Tree House in South London, sitting on top of the windowless concrete of a Civil Defence Control Centre. Hundreds of town halls, libraries and even schools built in the 1950s and ’60s retain their bunkers. Some are more classified than others. The warren beneath Chancery Lane was constructed in 1942 using non-English speakers to protect its identity. Codenamed “Kingsway”, it offered accommodation for up to 8,000 people but, in truth, was reserved for select individuals whose survival of a nuclear explosion was considered essential. Sections of this government “citadel” were taken over in 1944 by MI6’s opaque Inter Services Research Bureau, before being incorporated into the General Post Office’s equally shadowy “Scheme 3245” which saw the tunnels secretly extended beneath the city centre to form the hub of a deep-level, nuclear-proof communications network. At the peak of the Cuban Missile Crisis in 1962, on-site staff locked themselves in the tunnels for two weeks, dependent on underground facilities including the bunker’s own artesian well, before surfacing again. Kingsway formed a crucial part of the nation’s defence against Soviet attack, but no guided tours stop here, no information boards record the discreet heroism of those who manned it. Even with wars that did materialise, we do not like to dwell on fear, let alone paranoia. And this is a paranoia hidden behind the Official Secrets Act; the exchange’s classified status survived well into the 1990s. Like so much cold war infrastructure, it passed out of existence before ever fully passing into it.

A few years ago I began researching London’s secret tunnels for a novel I wanted to write, and soon became obsessed. I wasn’t alone: in recent years a shelf-worth of books on the capital’s buried networks have appeared, from Stephen Smith’s Underground London, Peter Ackroyd’s London Under, Fiona Rule’s London’s Labyrinth and, most recently, Nick Catford’s Secret Underground London, drawing on his involvement in Subterranea Britannica, an organisation studiously devoted to archiving the man-made underworld. Learning about subterranean London is a thrill akin to discovering a conspiracy theory in physical form: the familiar world gains a new dimension, one to which you now have privileged access. To know, as you cross Finsbury Park towards a horizon of Perfect Fried Chicken, that beneath your feet are the vaulted brick catacombs of an underground Victorian reservoir, or to see the office workers in Victoria, Westminster and Camden hurrying over the tops of nuclear-proof command centres, lends the everyday landscape a new depth of focus.

I was struck, as I began to discuss all this with fellow Londoners, by how many possess a fragment of the secret. An oral tradition mixes hearsay and urban myth, mapping speculation about the world beneath the streets onto the city’s explicit lines of power: was the Victoria Line really routed via Buckingham Palace to facilitate an emergency evacuation of Royals? Did a vent in the furthest stall of the ICA gents once grant you a glimpse of the crossroads beneath Trafalgar Square allowing senior officials to navigate the city centre without meeting daylight? One of my favourite rumours concerns Centre Point, the West End tower block that appeared in London in defiance of all planning permission before sitting empty for ten years. Could this be because it extends down as far as it rises? If so, does it connect to the Chancery Lane shelter, which stretches through Holborn to within a stone’s throw of the skyscraper? Conspiracy theories connect and mutate to encompass whatever lies in their path.

London’s subterranea is so abundant that it supports multiple tribes of devotees, some drawn to the cathedral-like beauty of Bazalgette’s sewers, some haunted by the tube’s abandoned “ghost stations” (one recent walking guide, Do Not Alight Here, navigates the city by their uncanny presences). Only one tribe makes the headlines, however: those whose mission is to see the underground world denied to them first hand. Contemporary “urbex” – urban exploration – emerged from a subculture of “drainers” (sewer walkers) and ruin junkies, devotees of the abandoned and derelict generally (the names of urbex websites often carry an air of apocalypse: “Silent UK”, “Contamination Zone”, “Whatever’s Left”). An American anthropologist, Bradley Garrett, has now published two books on the subject. Their titles convey a mentality: Explore Everything: Hacking the City and, more recently, Subterranean London: Cracking the Capital. The relatively recent concept of “place hacking” brings with it a concomitant sense of the city as unnecessarily encrypted. As with skateboarders, graffiti artists and squatters, using the city against the grain of its street-level ideology becomes self-consciously subversive. In the case of urbex, what began as the pursuit of adrenalin and cool photos became a political act; exploration became infiltration, and the goal was no longer just to reach the forgotten spaces beneath us but the forbidden ones, making tangible the unseen borders of the state.

The crew with whom Garrett explored got into sewers, disused tube tunnels and the abandoned Mail Rail that once transported parcels between London’s major sorting offices. But it was when they cracked the cold war domain of Kingsway that the Metropolitan Police’s Major Investigations Team began breaking down their doors. Thankfully photos survived, recording a world seemingly unchanged: the orange and brown 1970s décor of what may be the deepest bar in the world; medical posts with signage in 1950s fonts; enamelled switchgear; a canteen designed with the claustrophobic in mind, artificial windows showing bucolic landscapes; finally, the bombproof portals reminiscent of those on a submarine, leading to the extended tunnel network beyond. Discoveries like these are part of the attraction, of course: burial can be a means of preservation and there is always the possibility of stumbling upon messages from a world that has vanished in a generation.

Even as they were thrilled by the photos, a public with terrorist alerts on the mind did not always sympathise. “More than 300 of the capital’s most famous landmarks are being broken into at night,” the Evening Standard screamed as ASBOs were handed out and custodial sentences threatened. But an argument of utmost responsibility can be made. The first researchers to penetrate the underground defence network belonged to a generation for whom nuclear war was a living concern. In 1980 the New Statesman journalist Duncan Campbell lifted a manhole cover on Bethnal Green Road and, with his fold-up bike in tow, dropped down to spend the night cycling the deep-level tunnels. It was written up in a now famous article, “A Christmas Party for the Moles”, and expanded in a subsequent book, War Plan UK. Campbell defended his actions robustly. The government’s assertion that secrecy was necessary for our own protection didn’t stand up when it clearly wasn’t. He cited the minutes of a secret committee known only as MISC 379, meeting in 1951 when whispers of the tunnels first leaked. It would be embarrassing for word to get out, the committee concluded, for “either the public would think that the Government were out to protect their own skins and those of their immediate servants; or the public would assume that the shelters were intended for public use in time of war and would be disappointed when they found they were not.”

Campbell’s urban exploration belonged to a wave of interest now fading from memory, one concerned with piecing together the apocalypse we were forbidden to know about in order to demonstrate both the injustice and the insanity. In 1963 a group of anti-war activists, Spies for Peace, broke into Regional Seat of Government Number 6 (RSG-6), a bunker near Reading, where they photographed and copied documents. (Their only member ever to be publicly identified was Nicolas Walter, an anarchist and atheist, who went on to edit this magazine in the 1970s and ’80s.) In the event that Soviet bombers began their mission against the UK, these RSGs were to shelter representatives of all the central government departments, the purpose being to maintain law and order, communicate with the surviving population and control remaining resources. The “spies” published their findings in a pamphlet, “Danger! Official Secret RSG-6”, sending 4,000 copies to the national press, politicians (mostly in the dark about these things themselves) and peace movement activists, as well as distributing them on the CND march from Aldermaston. The pamphlet turned its sights on “a small group of people who have accepted thermonuclear war as a probability, and are consciously and carefully planning for it ... They are quietly waiting for the day the bomb drops, for that will be the day they take over.” RSG-6 was not a centre for “civil defence”, the activists stated, but one concerned with military government. Referring to emergency planning exercises in which the bunker had been activated, including a NATO exercise in September 1962, Spies for Peace asserted that they had demonstrated the incapacity of the public services to cope with the consequences of nuclear attack. The exercise, they said, “proved once and for all the truth of the 1957 Defence White Paper that there is no defence against nuclear war”. This was the backdrop for Labour-controlled local authorities such as Hackney declaring themselves “Nuclear Free” and refusing to partake in the preparations for war demanded by the government.

As the cold war fades into history the remains should remind us what creativity it demanded. In the 20th century, apocalypse became real. Nuclear war was first and foremost a challenge to the imagination, introducing a scale of destruction that demanded the entire nation be refigured. It was not only buildings and streets that led double lives: until 1992, thousands of men and women trained fortnightly as part of the United Kingdom Warning and Monitoring Organisation, ready to crawl down into one of 1,563 underground monitoring posts distributed across the country in order to provide the authorities with data about nuclear explosions and subsequent fallout. Public sector workers were also permanently on stand-by, whether they knew it or not: the bin man or park keeper who would turn up for work post-bomb to be told his new duties were those of a gravedigger; a director of social services ready to assume responsibility for refugee camps; a police sergeant who would have to round up Communist sympathisers to forestall domestic sabotage.

I met an astonishing number of people with direct experience of the nuclear state, from the ex-local councillor involved in installing a secure basement beneath Camden Town Hall, to the Royal Mail worker quietly indoctrinated into the truth of the off-limits passageways beneath their depot. If this was World War I or II there would be BBC documentaries and modules on the school curriculum. But the nuclear war that shaped our country isn’t just unique by virtue of its imaginary status; it remains intensely classified. Government papers enter the National Archives after 20 years, but the Lord Chancellor can retain anything connected to atomic defence indefinitely. There is an element of almost religious taboo - mankind’s possession of its own means of annihilation inspires no small amount of transgressive awe. But within the few files that have been released, a sense of something more bathetic can be felt: a part of all this imposed silence is embarrassment. Those responsible for preparing for a nuclear war were best placed to see the hopelessness. In a hint at the source of their information, Spies for Peace said that the recent simulation exercise had “convinced at least one occupant of one RSG that the deterrent is quite futile”. We couldn’t keep up with our own powers of destruction: new hydrogen bombs rendered many defences inadequate before they were completed. Senior officials debating the details of the “warbook”, the precise procedure for transition to war, recorded feelings of despair and disbelief. Sir Michael Cary, Under Secretary of State for Defence in the 1970s, asked whether he was really expected to perform “this nightmarish gavotte” on “the brink of Armageddon”.

We remember what we want to remember. Queues stretch up the Mall from Churchill’s wartime bunker. Less than two miles away, Kingsway is mothballed while the government seeks a private company with £5m to spare to take this awkward real estate off its hands. A war that never happens can’t be judged, but sometimes it needs to be recalled. In August last year both NATO and Russia resumed transition to war exercises. This year our government is committed to renewing Trident (four submarines, 40 nuclear warheads per sub). A party leader who states categorically that he would not launch a nuclear strike is dismissed as unfit for the role of Prime Minister. Naïve, we mutter, rushing past sealed hatches and rusting vents, the madness safely buried beneath our feet.