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Monday, 8:53 p.m.

Second Avenue and 77th Street

At Figaro Pizza, we eat — as Erling Kagge, the outdoorsman, puts it — our Last Supper: cheese slices and fountain drinks. The expeditioners are in a buoyant mood. Erling describes what he expects to see en route, a “negative beauty” formed by the absence of color, light, natural order. We will travel, he says, beneath the “culture of congestion.” A fascinating man. He’s a philosophical adventurer or perhaps an adventurous philosopher. Reads Hegel and tells tales of shooting polar bears in the Arctic. His comrade, Steve Duncan, meanwhile, is rebellious, with the wired energy of a perpetual undergrad. Smart, sarcastic, physically gifted (he looks like Owen Wilson). Our plan for tonight, he says, is to hike the Bronx sewers: “We’re going to be swallowed by the maw of the city.” One of Steve’s friends, a dude named Shane, will drive us to the drop site. I notice his T-shirt. It reads, “Trust me — I do this all the time.”

10:19 p.m.

East 172nd Street

Driving to the Bronx, Steve consults his laptop, checking our route on old sewer maps. We will descend with backpacks, headlamps, waders and a QRae II confined-space gas detector. Underground air is sometimes poisoned by carbon monoxide, hydrogen sulfide and explosive gases. Other dangers include sudden flooding. Because New York lacks absorptive earth, it can take as little as a half-inch of runoff, Steve explains, to cause a deluge. He doesn’t like that it’s snowing — windy bursts that skitter over the ground. Hopefully, he says, it won’t melt.

Tuesday, 12:36 a.m.

Exterior Street, the Bronx

We inspect our exit point — a manhole in the middle of the road. Will Hunt, a bespectacled 26-year-old who is writing a book about the underground (“The last frontier,” he says, “in an over-mapped, Google-Earthed world.”) will serve as our spotter. Will’s job is to watch for traffic: ascending from the hole, we do not wish to be hit by a car. We are to communicate by walkie-talkie. Will ties a long pink ribbon to the inside of the manhole cover. Dangling downward, this will be our signal we have reached the end.

1:20 a.m.

Van Cortlandt Park, the Bronx

Down we go by way of sewer pipe, joined now by Andrew Wonder, a shaggy former film student making a documentary about Steve. The change is stark, immediate: darkness, shin-high water, a dull ammoniac funk. My eyes adjust, and I see an endless tunnel, rounded, eight feet high and made of faded brick. The floor is scummy and perilous to walk on. Within seconds, Steve, Erling and Andrew rip their waders: they’re taking on water. We nonetheless progress and, after 50 feet, the entrance disappears. Forgot how much I hate enclosed spaces.

1:48 a.m.

Bronx sewers

Amazing. The sounds down here are even more impressive than the sights and smells: the Niagara-like crash of water spilling in from side drains; the rumble of the subway; the guh-DUNK! of cars hitting manhole covers overhead, like two jabs on a heavy bag. Steve says we’re only 12 feet beneath the surface, but it feels far deeper. The familiar world is gone: only sewage now, the press of surrounding earth, the anxious dance of headlamps on the water. Every 100 feet or so, an archway appears and we can see a parallel channel gurgling beside us with a coffee-colored murk. I shine my headlamp down and watch a condom and gooey scraps of toilet paper float by. I check the air meter constantly: no trace of gas, and the oxygen level is a healthy 20.9 percent. I ask Steve how he navigates down here; he laughs. “Hey, Erling,” he calls out, “you’re taking care of the navigation, right?” Funny.