A door designed to withstand an atom bomb swung open yesterday, and as a scruffy industrial lift rattled downwards, a hidden world under the heart of London finally gave up its secrets.

There is nothing hidden any longer about the mile of tunnels 100 feet below High Holborn and Chancery Lane, which BT, the most recent owner has just put up for sale.

But old habits die hard: this has been a twilight zone for over half a century - through the end of the war when MI6 moved in, in the years after when the Public Records Office stored 400 tonnes of classified documents here, and through the cold war, when the hotline linking presidents Eisenhower and Khrushchev ran through the most secure telephone exchange in Britain.

At the bottom of the shaft, behind a nondescript facade on Furnival Street, the only remaining entrance of the three original ways in, you turn right out of the lift into a comparatively small room - the only one still full of old switchboard equipment - then left into the mouth of a long straight tunnel. That meant that the street directly overhead must be ... "I can't tell you that," said supervisor Richard Bamford. "I know, but I'm not allowed to tell you. This place could become a secure or semi-secret store for a bank, say, and would they want you to know that?

"The people who constructed this place would not have been able to tell you where it was - they were not English speakers, they came from some European country, and they would not have known where they were working."

That last revelation was one of many stories, some probably true, some surely apocryphal, which emerged yesterday as former BT workers returned yesterday to guide the first outsiders invited into the tunnels.

In the 1980s the novelist James Herbert pleaded to be allowed in but was turned down flat: regardless he set the 1984 novel Domain, with its flesh-eating mutant rats, in a world instantly recognisable to the Kingsway Tunnel crew.

All the workers signed the Official Secrets Act when they joined: yesterday they felt free to tell stories which astonished David Hay, BT's official historian, who has pored over all the archives about the tunnels. He'd never heard about the foreign workers: he'd never heard that in the Cuban missile crisis the place was locked down for a fortnight, fully staffed, with maintenance crews and caterers, water from its own artesian wells, air filtered and recycled, and enough supplies to last for months.

John Tasker shook the door of his old office: it was locked. He came in the mid-80s as facilities manager, and stayed for eight years as all the equipment around him - including the original 1940s generators built in Lincoln, some still running yesterday - gradually became obsolete. "It was a strange place in winter, when you came down in darkness and went back up into darkness. I used to pick up the phone and call somebody up top to see what the weather was like, then at least you knew whether to take an umbrella."

Officially they never had a major incident: unofficially he recalls times when the cooling and ventilation system failed, and part of the complex became so hot that the pitch waterproofing the original bored tunnel melted and seeped through the chinks in the metal sections lining it. "Basically when you saw the engineers wandering around in shorts you knew you were in trouble.

"By then this place wasn't strictly secret - but it wasn't precisely not secret either. You just got into the habit of not talking about work to anyone outside."

Ray Gapes first came down in the lift in 1970 as an 18-year-old apprentice, and barely saw daylight again for seven years. "It was a fantastic place for a teenager, I found it very exciting. I explored every inch of it, I never actually asked for permission, I just went wandering around wherever I had the chance."

He knows Herbert was wrong: he never once met a rat.

At that stage the complex still had a fully stocked bar - with a notice warning staff to extinguish cigarettes, pipes and cigars before they returned to work - and a restaurant complete with mock windows with views of tropical gardens and pine tree fringed lakes, where as an apprentice he could buy a three course meal with a mug of tea for 2s 10d.

The restaurant was too expensive for him except on special occasions, so he took his breaks with the Tube 3 Tea Club, which was cheaper, more sociable, and half a mile less to walk, in a recreation room with a snooker table and a large tank of tropical fish which after a few years contained only piranha.

The complex began as two half-mile tunnels, dug by London Transport as the Blitz gripped London. They were built and equipped as deep level shelters for 8,000 people, and the plan was that London Underground would take them over after the war to extend the Central Line, whose trains rumble past in their own tunnels only a few feet away. Instead, part of MI6 moved in, a section cryptically known as The Inter Services Research Bureau, actually part of the Special Operations Executive originally set up to help resistance movements in Nazi occupied Europe.

When they left, on May 8 1945, they took every scrap of equipment and kit with them, and it is still not entirely clear what they were up to.

In 1954 the GPO took over the complex and added new tunnels, creating Kingsway telephone exchange, which became the London terminal of the transatlantic telephone cable, TAT 1.

Now the whole warren has almost been emptied, of tonnes of equipment - and a boat. "My mates thought I'd been down here too long when I told them about the boat," Bamford said. It was another of Kingsway's secrets, a small boat stored in case of emergency flooding at another exchange, despite the slight difficulty of getting one up from 100 feet underground if floods bad enough to need its help hit central London.

Now only a handful of maintenance staff walk the tunnels every couple of days. "I've never heard of a ghost story about this place," Malcolm Solairi said, "it's actually very peaceful down here, I'll miss it."