It must have been the third or fourth day -- time, by that point, had started to dissolve -- when I stood in camping gear on Fifth Avenue, waiting as my companions went to purchase waterproof waders at the Orvis store. We had already hiked through sewers in the Bronx, slept in a basement boiler room, passed a dusty evening in a train tunnel; we were soiled and sleep-deprived, and we smelled of rotting socks. Yet no one on that sidewalk seemed to notice. As I stood among the businessmen and fashionable women, it dawned on me that New Yorkers -- an ostensibly perceptive lot -- sometimes see only what's directly in front of their eyes.

I suppose that's not a bad way to think about the urban expedition we were on: a taxing, baffling, five-day journey into New York's underground, the purpose of which, its planners said, was to expose the city's skeleton, to render visible its invisible marvels. The trip's conceiver, Erling Kagge, a 47-year-old Norwegian adventurer, had ascended Mount Everest and trekked on foot to both the North and South poles. His partner, Steve Duncan, a 32-year-old student of public history, had logged more than a decade exploring subways, sewers and storm drains. Last month, the two of them forged a new frontier: an extended exploration of the subterranean city, during which they lived inside the subsurface infrastructure, sleeping on the trail, as it were.