Cites are built over unfathomable labyrinths: but it is a rare imagination that can make us see that. In this ambitious new novel, the Irish writer Colum McCann burrows under the familiar surface of New York and finds newness there - creating a story of pained, powerful complexity and dark life.

McCann's fascination is the world underground. In a narrative that begins in 1916, he traces the story of Nathan Walker and his fellow "sandhogs" - men who dug the tunnels of New York's subway system. Nathan, a black man, works in a tight multicultural band with Vannuci, Power, and Con O'Leary, until a dramatic blowout (based on fact) when the men are sucked up from the tunnel through the East River and sent back into the air on a geyser of water. O'Leary dies in the accident, his trapped body one of many corpses on which the proud city is built.

Alternating with Walker's progress through the century is the modern account of a man known as Treefrog who lives among the Moles: people who have made homes in the tunnels, surviving on lightless air and "redemption money" (from cashing in cans and bottles) and finding ways to cohabit with each other and the rats. Treefrog was once a confident construction worker building the city's skyscrapers, but a moment of misplaced lust with his daughter causes his banishment and turns his mind to a muttering mess. He moves around the tunnel rafters busy with fixations on maps and symmetry and self-mutilation; he wants "to murder his hands in shame".

McCann takes the risk of drawing places he knows we haven't been. Both territories feel over-researched at times; while anachronisms sometimes clutter the characters' diction. Walker, especially, can sound more wished- for than real, as he plays a part in McCann's account of American race relations that courses through the novel like a troubled river.

But eventually Walker, and Treefrog, become their own strange, alive selves, as does the city itself. As in his collection of stories Songdogs, McCann writes a careful, idiosyncratic prose - "a street cantankerous with car horns"; a widow who "remembers herself to sleep" - with a rugged lyricism that brings Cormac McCarthy to mind.

The novel is not so much plotted as built, as Walker's life moves on through his marriage to O'Leary's daughter, Treefrog's memories move back, and we begin to see how the stories, like the two ends of a tunnel, will eventually meet. It is a clever and moving structure, and the moment of meeting is a melancholy revelation.

Unfortunately it leaves McCann uncertain where to go, and he brings the story to an overly heightened, symbolic close. But his rich infusion of language and sympathy make you feel that McCann understands tunnels, and exiles, and the mad scrape of solitude. Together they make This Side of Brightness a lasting, haunting work.