At twelve-forty-five on Thursday night, unable to sleep, the climber Alex Honnold got out of bed, picked up his backpack, and walked across the street from his hotel in Jersey City, New Jersey, to a sixty-nine-floor luxury apartment building called the Urby. Honnold hid his Candlewood Suites room key and his flip-flops under a fence. “Hope no one takes them,” he said, shrugging. He ate some dark chocolate, “to get amped.” Then he grabbed hold of a grate at the base of the building and, with no fanfare or dramatic final words, began heading up the building’s “northeast ridge,” as he called it, without a harness or rope.

A boyish thirty-three-year-old Berkeley dropout who often sleeps in a Sprinter van, Honnold had donned brand-new climbing shoes and applied chalk to his large hands. It had rained a few hours before, and the forecast was only getting, in his words, “more grim.” He added, “If it starts to rain again, I’ll just knock on a window, you know, and get the resident to call somebody.” Rappelling back down wouldn’t be an option. Climbing this way, without safety gear, is called free soloing. Honnold is the greatest climber of this kind in history. He earned that title last summer, when he free-solo-climbed the three-thousand vertical feet of El Capitan, perhaps the most famous and beautiful rock face in the world, in less than four hours. (The first team to ascend the route, in the nineteen-fifties, took forty-seven days.) The Times called it “one of the great athletic feats of any kind, ever.” Afterward, Honnold hiked back down a trail and executed a few rounds of finger pull-ups in his van. (A National Geographic film about the climb, “Free Solo,” premières this weekend, at the Toronto Film Festival.)

On Thursday night, Honnold took his time, enjoying the nocturnal view of the city as he ascended the Urby’s first nine floors. The building, constructed two years ago, is a cantilevered tower that resembles a series of hastily stacked blocks. His initial route traversed the perforated metal enclosure of the building’s parking garage—its walls were full of “nice, secure holds,” Honnold said—up to an elevated terrace where, he later told me, “I fully hung out, took my shirt off to get cool.” He spent ten minutes there, watched people smoking below, and further assessed the building. It wasn’t the climbing that worried Honnold, at this point, but the windows above, where he could see lights turned on.

He wasn’t the only one with this concern. A few hours earlier, he’d discussed his plan with David Barry, the building’s developer, who had agreed to let Honnold use the Urby last month. (Barry’s lawyers finally signed off on the plan on Wednesday, after Honnold signed a waiver, lease, and indemnification agreement.) Barry had expressed mild concern about alarming his tenants, whom he’d decided not to notify ahead of time—per Honnold’s request—in order to avoid a scene. “I’ll be quiet and discreet,” Honnold assured Barry, “as long as they don’t punch through the window.” He added, “I’m not worried about people compared to stuff I deal with in nature—like when a bird comes down your arm. Or mice, or scorpions.”

Still, Barry’s team had a contingency plan. Alexander Waxman, Urby’s creative director, explained to me what would happen if a resident were to “freak out” as Honnold climbed past a window. “We’ll tell them, ‘Everything is fine. We’re aware of it. We’ll follow up with more information later.’ ”

Many who free-solo at the highest level—John Bachar, Dwight Bishop, Ueli Steck—eventually fall to their deaths. Others have managed to hold on: Alain Robert, a skyscraper-obsessed Frenchman, whom Lauren Collins wrote about in 2009, is now in his mid-fifties. Honnold has voluntarily had his brain examined, curious whether his ability to hold himself together perched thousands of feet off the ground on marble-size nubs and flakes of rock is indicative of some neurological abnormality. His emotion-processing amygdala is intact, it seems. However, an MRI scan, conducted a few years ago, showed that it was less active in conventionally frightening settings than the typical amygdala is.

Honnold can seem introverted, and describes himself as “not very warm,” but he insists that he does not have a “death wish.” Most of his climbing is roped. His free-solo attempts at El Capitan and Moonlight Buttress—a twelve-hundred-foot rock face in Zion National Park—were undertaken only after careful planning and study. (In the case of El Capitan, he practiced with safety gear for more than a year, memorizing each move.) I recently wrote about the first scouting trip that Honnold took to the Urby, a few weeks ago, during which Jersey City’s mayor, Steven Fulop, watched Honnold practice part of his route. Mayor Fulop confessed to me that he’d been drunk when he first suggested that Honnold climb his friend Barry’s apartment building, after the two of them happened to meet at the exclusive Yellowstone Club, in Montana. “But I’m still on board,” Fulop said.

“I differentiate between risk and consequence,” Honnold told me. “Sure, falling from this building is high consequence, but, for me, it’s low risk.” Then he shrugged.

Once Honnold left the building’s ninth-floor terrace, the route became more difficult: he pulled himself up by windowsills and frames, and perched on ledges the width of a fork, trying not to stare into the blinding accent lights below or dislodge the occasional security camera. He moved at what he called a “casual” pace, taking a few minutes per floor. The ledges were wetter than he’d expected. He re-chalked his hands with a pouch tied to his waist and kept moving. “It was weird,” he said later. “There were lots of people still awake at, like, one-thirty in the morning on a Thursday night.” He climbed laterally more than he’d expected, to avoid them. “I didn’t want to be encroaching on peoples’ scene,” he said. His foot slipped at one point. “I was, like, ‘This is kind of dumb,’ but there’s no real better time to do it.”

He passed people sleeping a few feet from him and tried not to bang his feet against their windows. He quickly scurried past a man who might have been undressed. He saw people watching television, staring at their computers, and doing other mundane things. “One dude took a selfie with me at one of the windows on the twenty-first floor,” he said. “I gave him the thumbs up. It was kind of a weird deal. His girlfriend seemed deeply uncomfortable.”

Honnold stopped at the twenty-fourth floor, about an hour into the climb, where a balcony jutted out far enough to give him room to recline. “There was no way to go any higher without navigating over someone’s living room, and they still had lights on,” he said. “I decided that I was done feeling like a Peeping Tom.” So he took out his phone and texted Barry to ask if someone would let him inside whenever convenient. He removed his shoes. Then he called his girlfriend, on the West Coast. He tried to stay out of sporadic rain for the next three hours. He caught up on news. “I read the Kavanaugh-confirmation stuff,” he said. “And that Times Op-Ed. What a shit show, this Administration.” Lightning struck in the distance. “It was kind of epic,” he said. To stay warm, he huddled near the exhaust coming out of an air-conditioning vent. “I also did my daily core workout,” he went on. It was the most time he’d ever spent staring at a city. He doesn’t spend much time in urban environments. “It’s kind of crazy how much light there is in the middle of the night,” he said.