You know those dreams, where you find yourself in a familiar house only the house contains more rooms than you ever thought, and rooms come off rooms, passages open out into chambers and before you know it you have strayed somewhere else altogether?

London can be like this. Last week, I walked though a bomb-proof door on Furnival Street in Holborn and took a lift 100 foot down to a vast subterranean world that few Londoners ever suspected existed.

These are the Kingsway Tunnels, a secret underground bunker over a mile long that present owners, BT, have just put up for sale. Built originally as the Chancery Lane public bomb shelter in the Blitz, the lair was taken over by MI6 until the end of the war, before the Public Record Office took residency and finally GPO (BT as it was then known) bought it up and extended the network of chambers and passages in 1956.

It became the Kingsway telephone exchange, the most secure in Britain. Broken circuit boards and a framed Factories Act of 1944 bears witness to it now - it's like a vision of a future that never was. Rumour has it that MI6 sealed it off, manned, for a fortnight during the Cuban Missile Crisis - but fact recalls that until the mid-Nineties some 150 telephone engineers worked here daily. There was a snooker hall, a canteen adorned with pictures of palm trees, a brown, tube-shaped bar and a tropical fish tank.

Still, it remained secret. The supervisor who led our tour claims those who built it were shipped in, spoke no English and had no idea where they were working - a precaution. Even now, displaying a level of secrecy that suggests the war never ended, he refuses to reveal exactly which street it lies under. An electrical diagram on the wall for some reason incorporates the old newspaper offices on Fleet Street. Was this place used for wire-tapping journalists? No one will say.

Of course, the temptation is to sneak off away from the press corps and explore the tunnels by myself - "Oi! Don‘t open that door!" I am told, though, whenever I slip away. Or is that a smoke screen, a red herring? How many more such places are there in the bowels of London.

Tar seeps through the walls. Central Line trains rumble past, unnervingly close. It will take any prospective buyer at least another £5 million to render the space suitable for commercial use: storing bullion, say.

I say the Punchdrunk theatre company should turn it into a living art installation. Or an eccentric billionaire sould perform weird fantasies down here. Or Dr Strangelove's vision for a post-nuclear society could be acted out!

Excuse me. Must be the lack of daylight.