But there is a positive side, too: because of the need to accommodate the dramatic fluctuations in the volume of flow, the main New York sewer pipes are large, and on dry days are typically filled to less than 10 percent of capacity. This means that there is room for Duncan to wade through. He was not going to do that today, but he was clearly preparing for a mission.

Some practical points. When he goes in he wears chest-high waders. He is aware of the danger of sewer gases and takes measures to protect himself. Normally he comes out the way he goes in. This is because the access points are typically manholes with manhole covers on the streets. Opening a manhole cover from below without knowing where it is and what’s coming is a very bad idea. Scouting for just the right manhole is therefore an important and time-consuming job. Part of the survey process involves peeking through manhole covers, or briefly prying them open on streets that are too busy to allow full access. When doing this it helps to wear an orange vest—not to keep from getting hit but to wave to the world like “Hey! It’s O.K.! I’m not trying to hide! I’m not dangerous!” I asked, “So how do you keep from getting run over?” He said, “Oh, I wait for the red light, and then I run away when the cars are coming. I don’t have a lot of science to this stuff.” I asked, “How about some cones?” He said, “Have you ever tried to carry around a half-dozen full-size traffic cones? Those things are heavy as fuck.” He thought about it. He said, “I have the miniature cones.” He has a miniature manhole hook as well.

More of the practicalities: No monsters lurk in the sewers. There are very few rats, because they have difficulty getting a toehold. Sometimes there are eels that swim in at high tide during storm-water outflows. Duncan’s girlfriend is squeamish about them. He discovered this when he once took her down into a sewer. She is a freelance writer who frequently works for Le Monde diplomatique, in Paris. Duncan himself does not like the feeling when eels bump against him in the dark. By comparison, he has nothing against the sewage itself. It’s not that he likes it, but he is impatient with what he considers to be the irrational prejudice of others. He said, “Even undiluted sewage is more diluted than you would think, because it includes all of our daily water-use stuff.” I was doubtful. We had been peering down into a sewer that looked pretty thick. He made a visible effort to be polite. He said, “It’s fucking miserable to go through 19th-century sewers. I’ve been through such four-foot tunnels for about a half a block. Of course it’s not at all un-doable. If you or I were locked away for life in some dank dungeon, and we were trying to get out, a four-foot-diameter tunnel would be great. We could do it in a two-foot-diameter tunnel. But in terms of, like, an enjoyable time wandering through a tunnel of our own volition, it’s much better to be able to stand up.”

I insisted on my prejudice. “Well, I imagine it would also be better if the tunnel didn’t have shit in it.”

He said, “Yeah, yeah, yeah, that too.” He paused. “That’s the other problem with the small ones. It’s you can’t, um … ”

“Stay away from it?”

“Yeah.”

One last practical point: it is important to watch not just the weather but the tides. A few years ago Duncan and a friend set off on an expedition up a big sewer in Queens. They went some distance in, and as they turned around to head out they noticed that the current had reversed and the water was rising fast. It was the tide coming in—and belatedly they saw indications that it regularly filled the tunnel to the top. With the water rising above their waists, and unable to fight the current for the return, they argued about the course of action to take. Duncan believed there was no time to gamble on manhole covers they might come upon, because so many of them in his experience have been welded shut by the battering of traffic; he believed they should ride the current as far up-sewer as they could get, and at the last moment find a manhole, climb the ladder, and tie themselves there if they couldn’t open the cover—so that if the water kept flooding upward and drowned them while overflowing onto the street, their remains might someday be found rather than being flushed out to sea. His friend disagreed and insisted on an immediate attempt to escape through the next available manhole. They came to one, climbed the ladder, and were unable to budge the cover. Down they went again into the rising water and proceeded half swimming in maximum haste to a second manhole—where again they could not lift the cover. On they went to a third manhole, and up the ladder, and this time the cover gave way. It was night outside, and raining. They emerged with their packs, ropes, and headlamps, and sprawled onto a quiet residential street as a woman in a minivan drove by looking at them and shaking her head in disbelief. Kids these days. But Duncan was not a kid anymore. He swore never to make such a mistake again.