The bridge rose up and away from the city's northwest quadrant, spanning the great Yangtze river. And yet, from the on-ramp where the taxi let me off that Saturday morning, it seemed more like a figment of the imagination, a ghostly ironwork extrusion vanishing in the monsoon murk, stretching to some otherworld. It was disorienting to look at, that latticed half-bridge leaving off in midair, like some sort of Surrealist painting. It gave off a foreboding aura, too, untethered and floating, and yet it couldn't have been more earthbound—and massive. Later I'd find out it was made from 500,000 tons of cement and 1 million tons of steel. Four miles long, with four lanes of car traffic on the upper deck and twin railroad tracks on the lower, it transferred thousands of people and goods to and from the city every day. But now the clouds clamped down, and a sharp scent of sulfur and putrid fish wafted on a dank puff of air. Rain slithered from the sky. There, before my eyes, the bridge shimmered and disappeared, as if it had never been visible in the first place.

Its formal name was the Nanjing Yangtze River Bridge, and it served one other purpose for the masses: At least once a week, someone jumped to his or her death here, but a total was hard to come by, in part because the Chinese authorities refused to count those who missed the river, the ones who'd leapt and had the misfortune of landing in the trees along the riverbank, or on the concrete apron beneath the bridge, or who were found impressed in the earth like mud angels, two feet from rushing water. Perhaps such strict bookkeeping came in response to the fact that China already posts the highest sheer numbers, about 200,000 "reported" suicide cases a year, constituting a fifth of all the world's suicides. For a long time, the Communist government simply ignored the problem, hoping it would go away, or maybe thinking in the most Darwinian terms of suicide as its own method of population control. One recent case highlighted just how the Chinese bureaucracy tended to deal with prevention. In the southern city of Guangzhou, workers had been ordered to smear butter over a steel bridge popular with jumpers, in order to make it too slippery to climb. "We tried employing guards at both ends," said a government official, "and we put up special fences and notices asking people not to commit suicide here. None of it worked—and so now we have put butter over the bridge, and it has worked very well."

In Nanjing, the bridge remained butterless, even as the city spit out its victims. Nanjing was now just another one of your typical 6-million-person Chinese metropolises, one of the famous "Three Furnaces" of China because of its unremitting summer heat. Daytime temperatures regularly topped ninety degrees here—due to hot air being trapped by the mountains at the lower end of the Yangtze River valley…and, oh yeah, because all the trees had been chopped down—and the sun rarely shone. Meanwhile, the city continued to explode in the noonday of the country's hungry expansion. The past was being abandoned at an astonishing rate, the new skyscrapers and apartment buildings replacing the old neighborhoods. Everything—and everyone—was disposable. Schisms formed. The bridge loomed. Loss led to despair, which, in turn, led to Mr. Chen.

I'd come through thirteen time zones just to see him. Once free of the taxi, I began trudging, a quarter mile or so, the bridge trembling under the weight of its traffic, piled with noisy green taxis and rackety buses, some without side panels or mufflers. Unlike the suspended wonder of Brooklyn or the quixotic ponts of Paris, this couldn't have been mistaken for anything but stolid Communist bulwark: At its apex, the bridge was about 130 feet above the water; was built with two twenty-story "forts," spaced one mile apart, that from a distance had the appearance of huge torches; and contained 200 inlaid reliefs that included such exhortations as Our country is led by the working class and long live the unity of the people. A brochure claimed that the bridge was both the first of its kind designed solely by Chinese engineers, and also "ideal for bird-watching." People teemed in both directions. Umbrellas unfurled, poked, and were ripped from their rigging, leaving sharp spiders dangling overhead. As I registered the passersby—their eyes fid downward—everyone seemed a candidate for jumping, marching in that mournful parade.

He was close now. I could tell by the banners and messages—some were flags, some were just scraps of paper—that fluttered earnestly from the bridge. Value life every day, read one. Life is precious, declared another. His cell-phone number was emblazoned everywhere, including little graffitied stamps he'd left on the sidewalk, ones I tried to decipher beneath the blur of so many passing feet. And then Mr. Chen came into view, conspicuous for being the only still point in that sea of motion…and the only one sporting a pair of clunky binoculars, the only one watching the watchers of the river.

The city continued to explode in the noonday of China’s wild, hungry expansion. Everything— and everyone—was disposable. Loss led to despair, which, in turn, led to Mr. Chen.

He stood at full attention at the South Tower. Perched off one side of the tower was a concrete platform surrounded by Plexiglas, a capsule of sorts where yawning sentries did their own dubious monitoring of the bridge through a mounted spyglass, as if conducting a sociological study at a great remove. The sentries looked like kids, while Mr. Chen, who stood out front on the sidewalk, among the people, looked every bit of his forty years. He had a paunch, blackened teeth, and the raspy cough of an avid smoker, and he never stopped watching, even when he allowed himself a cigarette, smoking a cheap brand named after the city itself. He wore a baseball cap with a brim that poked out like an oversize duck's bill, like the Cyrano of duck bills, the crown of which read "They spy on you".