Listen; to the pictures

In a giant cavern called the Cathedral, deep in the bowels of the Clink Street Vaults, Robert Wilson, his sound collaborator Hans Peter Kuhn and an army of technical experts are all waiting for the 6.15 from Dartford. Kuhn has rigged up a battery of microphones on the track that runs over the building so that when trains pass along the top of the arch, a tidal wave of sound surges through the vaults beneath. Everything is in place for the soundcheck except the train. It's late.

Only after 15 minutes is a tiny rumble heard in the distance, soon building up to a deep torrential growl, its bass registers bloated out of all recognition. Then, as it passes overhead, an ear-splitting howl shakes the building as 200 tons of metal along with a couple of hundred bored, shuffling commuters unwittingly take part in Robert Wilson's and Hans Peter Kuhn's HG. "I just hope we don't hit a rail strike," mumbles Kuhn on the way out, "it would ruin my best idea in the whole piece."

Gaining admission into the hallowed HG ground was a suitably monumental task. For an entire week - about as long as Wilson's KA MOUNTAIN and GUARDenia TERRACE, a piece which clocked in at 168 hours - I pleaded, dissembled, demeaned myself for the privilege of being permitted to observe Wilson and Kuhn at work. Producers Art Angel are understandably nervous of outsiders, not only due to the phenomenal scale of the HG project, but also because Wilson's reputation is only beginning to blossom in this country following last year's desiccated, formalist production in Edinburgh of Gertrude Stein's Dr Faustus Lights the Lights. In almost every other European country, Wilson has long been recognised as one of the giants of 20th-century experimental theatre and design. His work is characterised by an almost complete lack of conventional narrative, character, plot, tension and all those other redundant qualities we have come to expect from theatre. Instead they are replaced by vast, dream landscapes, glacial choreography, fragmented texts and timespans of inordinate lengths. His visionary masterpieces have included: Einstein on the Beach, a five-hour, non-narrative opera created with Philip Glass conjuring up unlikely images from the life of Einstein; The CIVIL warS, a projected 24-hour extravaganza of American history; and Deafman Glance, the seven-hour silent "opera" that kicked off his career: "I never saw anything more beautiful in the world since I was born," commented surrealist founding father Louis Aragon after the 1971 Paris opening. "It is what we who fathered surrealism dreamt might come after us, beyond us."

Two years in development, HG comes at a time when Wilson is increasingly escaping from the confines of the stage and moving into more unruly spaces. At the Venice Biennial last year, his installation, Memory/ Loss, which was located in an old salt warehouse, picked up a Golden Lion for Sculpture. A site for HG - a piece inspired at some distant Wilsonian remove by H G Wells's Time Machine - has been found in the old, bonded warehouse on Clink Street, an unimaginably vast labyrinth of resonant chambers, reverberating annexes and dark velvety corners that was once a medieval dungeon for the nearby Clink Street prison.

HG begins in a Victorian drawing room and then takes the visitor on a time journey through Wilson's idiosyncratic obsessions. The connection with Wells seems obscured and buried at times, even to Wilson. "I'm not sure if it's about H G Wells," he comments enigmatically. "I think it's about Hamlet's ghost."

In the past two weeks, Wilson has already transformed the mouldering space into a playground of the imagination, into his own imagistic purgatory: sphinxes peep out of alcoves; rooms are strewn with clogs, slippers, stilts, ballet pumps; light bulbs drip from the ceiling. When I arrive, a man from Animal Ark, a company that supplies animal props, is confronting Wilson with his weird menagerie of evolutionary aberrations to see if they meet with approval. He reveals a gigantic elephant's foot, an outsize bovine skull, and a stuffed, black raven that looks as if it has just received a liberal application of brillcream. "This is all from my own private collection," he beams.

Wilson's decisions are based entirely on his intuitive response to the elements around him. His thoughts - as he demonstrates by constantly snatching my notebook and answering questions in spindly diagrams and hieroglyphics - are exclusively in mental pictures. "I look at the architecture of the space just to see and feel what is there," he says. "I get my ideas from the space: the space is low or the space is high or the space is damp or the space is dry. I simply respond to what I can see." As in his theatre work, Wilson aims to clear away the interpretive clutter so that there is no distraction to the direct sensory response. "Just enjoy the scenery," he advises, "the architectural arrangements in time and space, the music, the feelings they all evoke. Listen to the pictures."

At this point, Art Angel co-producer Michael Morris arrives with the news that he has discovered a pocket watch, five feet in diameter, in the window of a jeweller in Smithfield. "I don't know if it might be too obvious," he says. Immediately, Wilson is whisked off to take a look at the object. "It's always this chaotic," says the other half of Art Angel, James Linwood. "There was another room scrapped today. And we don't know what to do with two more."

By comparison, Kuhn's approach is civilised and sedate; he briskly wanders around putting the sound system in place. His collaboration with Wilson which began in 1979 with DDD has reached such a level of understanding that "there is no need to discuss much". The sound environment is created on much the same basis as the visual imagery: "It's very irrational," he says. "It's like making a happening in the sensorium rather than in the rational part of the brain. I use the sound to create states and not to illustrate points." A little later, he takes me to a low-ceilinged room from which a rectangle had been sawn out so that it is possible to see fluffy clouds dangling above and a small gaudy globe. Kuhn's idea is to have the sound of footsteps - wonderful, crunchy bites of sound - circling the public below as they peek into the ether. "I have my own sound library," he says. "This footstep I found in the aisle of a theatre in Lausanne. It was astonishingly loud, creaky."

Later in the day, Wilson returns after rejecting the outsize timepiece. He is not in a good mood. "I don't want to talk about the work when it's in progress," he drones at me, filling up the chamber with his syrupy Texan monotone, "and I don't want to have to say this again. I thought people weren't supposed to get this far into the building." The fly on the wall is about to be swatted. But before there is an opportunity to ask me to leave, the first full soundcheck rips through the vaults giving some sense of the devastating potential of the piece. A great web of sound snares the images and turns Wilson's simple configurations and objects into living things.

On my way out I come across the man from Animal Arc in the courtyard, a gigantic blue shark balanced precariously on his shoulder. He loads the raven and the elephant foot into the back of his truck. Someone has broken the sad news to him that his menagerie didn't meet with Wilson's approval. Up close, I can see that the shark's fin is held on by a large band-aid. "I don't know why Mr Wilson didn't like them," he says, flashing a wan, wounded smile. "When I showed them to him, he didn't say anything at all." But then, he rarely does.

n Clink Street Vaults, SE1 (0171-336 6803), Tues to 15 Oct: 4-8pm Tues to Fri 12 noon-8pm Sat & Sun