There Are Times when a programme booklet tells you more than you want or need to know. Merce Cunningham's Ocean, we learn, is composed according to the number of hexagrams in the ancient Chinese oracle, the I Ching - 64. John Cage's orchestral accompaniment, completed after his death by Andrew Culver, consists of 32,067 musical "events" spread over 2,403 pages divided between 112 musicians. The dance itself divides into 19 sections derived from 128 dance phases processed by a computer. Imagine your critic's predicament when, only seconds before countdown at Belfast's Waterfront Hall, the sweet elderly lady in the next seat declares: "I love modern dance. Tell me, is there a story?"

The numbers are not the story of course. But Merce Cunningham so scrupulously avoids any glimmer of narrative that you're stumped to say how or why his dances exert their mesmerising pull. Ocean - the final, giant collaboration between the legendary American avant-gardists Cunningham and Cage - also claims some mysterious connection with the novels of James Joyce, which makes it fitting that the Belfast Festival has scooped the European premiere but does nothing to help audience comprehension. Cannabis would help, it occurs to me, but the legal option is to settle well down in your seat, slow your pulse, and forget everything you've ever seen or known.

A solitary male dancer in a blotchy blue skinsuit spirals to the centre of the circular stage, erect as a spindle on a loom, yet pliant as cloth. A girl in deeper blue comes on at a run, but instead of eagerly linking up with him as you'd expect, remains self-absorbed and separate, observing the symmetry of her own deep, deep plies as she flexes her knees with miraculous slowness, or holds an arabesque on the ball of one foot for what seems a pensive eternity. After many minutes more blue figures appear, each spinning along his or her own planetary path, or massing and dispersing like schools of slippery fish.

Both individual movements and broader patterns have a dreamy, remote, almost Zen-like quality, the overlapping phrases punctuated only by changes in the figures' hue through violet and tangerine and back to blue. Marsha Skinner's elaborate lighting rig hovers over a huge circular mesh of fishing-net, suspended above the dancer's heads, which causes the coloured beams to puddle on its surface, much as you imagine sunlight would look from an ocean bed. Monet would have ditched his garden for this.

The 112 musicians (valiant students from Belfast's conservatoire) bunch in small groups in the highest, furthest reaches of the Waterfront's vast circular gallery and doggedly count their way through the chance-driven score: five groups playing sequences of 19 short pieces simultaneously but each in a different order, moving on to the next sequence just as and when they're ready. Yet the result of this semi-random jam session is not complete cacophony. At the lower end of the dynamic range, pitches blur into a generally pleasing hum through which David Tudor's ambient electronic sea noises can burble unimpeded. You hear the moaning of whales, sonar ship noises, sounds that could conceivably be the movement of Arctic ice. And even though these sounds have been tinkered with, they make a refreshingly unambiguous nod to the work's title. Without them, the audience really would be at sea.

By allowing chance to enter the choreographic process Cunningham believed he was tapping into the mysterious functions of the universe. And it's in this grander, metaphysical sense that the dance does illuminate the oceanic theme. Magnificently. It seeps into your consciousness over the long 90 minutes 00 seconds of the work's duration, a time-scheme rigidly underlined by five vast video monitors which act as stop-watches throughout. This dance has absolutely nothing to do with emotion. Yet the rigorous focus on physical detail, and the superb skill of the dancers, actually seem to uncover some super-expressive core of movement itself - an absolute of Creation.

It was bold of Belfast's new festival director, Sean Doran, to spearhead his programme with such a difficult work. Other European cities have made a bid for Ocean, but not had a venue-in-the-round to stage it. Belfast's gleaming new Waterfront Hall - the most beautiful modern theatre I have ever seen - might have been built for the purpose. Its great curved foyer spaces were well exploited, too, in a pre-theatre showing of John Cage's Roaratorio. This requires the audience to do the moving around while the performers (in this case scores of loudspeakers, strung out like pylons across the landscape of elegant bars and foyers) remain static. Seagulls, Irish pipes, hymn singing, factory noise, babies crying ... more than 2,000 separate sounds are arranged as a polyphonic collage, which varies in intensity as you roam between the speakers. "In the beginning was the sounddance", wrote James Joyce in Finnegans Wake, and this is Cage's mind- stretching realisation of what that might be.

Jenny Gilbert went to Belfast courtesy of the N Ireland Tourist Board.