Climber Alex Honnold’s feat—ascending the 3,000-foot rock face of Yosemite’s El Capitan without ropes—is one of those spectacular human achievements that almost defy belief. But photographic evidence exists proving it really happened, in the form of the documentary Free Solo, one of the biggest nonfiction hits of this or any year.

The film was directed by Elizabeth Chai Vasarhelyi, a non-climber, and her husband Jimmy Chin, who is not only a climber but an elite one, and therefore in a position to judge the scale of what Honnold accomplished.

“I’ve been filming in this space for the last 20 years, and shooting stills, and really have worked with the best athletes, often at the peak of their careers,” Chin remarks. “And even given that much time and experience I’ve had doing this, there’s nothing I can compare it to. It’s truly one of the great human physical and mental achievements, in my opinion, of our time. If not more than our time.”

Free Solo ties audiences into anxious knots as they witness Honnold’s upward journey, knowing at any moment, with any misplaced move, he could plunge to his death.

“Almost everybody I talked to who’s seen the movie says the same thing: ‘My hands were sweating the whole time,’” Chin tells Deadline. It was a similar feeling for Vasarhelyi and much of the filmmaking team: while Honnold had nothing to grasp but rock, they were on tenterhooks.

“We had practiced for so long, suppressing our own emotions around it, because we had to insulate Alex from any pressure. So, we’d be freaking out in the background, but you can’t bring that, right?” Vasarhelyi comments. “You don’t want to be responsible for adding that one thing of pressure, because you’re sweating.”

There are elements to the climb moviegoers did not see, such as how Honnold maintained his energy when he had nothing on him to eat as he started up the mountain. Turns out, he had planned ahead, storing necessaries in advance in nooks and crannies.

“While he was training, he’d left bits of food, and bits of water,” Chin reveals. But there were creatures to consider who would have enjoyed partaking in a snack.

“If you don’t package it correctly, birds will peck through,” he notes. “If it’s just a plastic bag, they’ll peck through and eat it. The ravens are extraordinarily good at it. And they’re well-trained because there’s so many climbers. But you put it in these bird-proof cases.”

National Geographic/Jimmy Chin

Honnold also got to know who was hiding in the crevices, lest a surprise encounter send him careening.

“Every time he goes by [one spot] the same bird is always there,” Vasarhelyi mentions. “So climbers know that when they put their hand in, ‘There’s where the three bats live. And here’s where the bees are.’ It’s just the same thing.”

The filmmakers left out some mundane—but relatable—moments, as when nature called with no facilities in sight.

“He went to the bathroom,” they note, adding that wasn’t the only break for Honnold. “He took his shoes off and hung out at some point,” observes Chin. Vasarhelyi continues, “Because the shoes are so tight you need to take them off, feel your feet a moment.”

Feelings of a different sort play into Free Solo—the directors documented Honnold’s very tentative journey to allow a romantic partner into his life. He appears almost Spock-like—on much safer ground with logic than emotion.

Free Solo
National Geographic

“He had this woman who entered his life who was self-confident enough, and emotionally intelligent enough to believe in herself, and push back on him, but also have this premise of, ‘You are who you are, and I love you for that,’” Vasarhelyi says. “So that the greatest solo of his life ended up being more about connecting, in some way.”

Vasarhelyi and Chin have been living their own love story. They met initially while collaborating on what became the hit 2015 documentary Meru, another film about an intensely-dangerous mountain ascent. Now they have two children together, one of whom was born during the making of Free Solo.

“I remember telling Alex that I was pregnant. And he was like, ‘Great.’ I was like, ‘You have no idea what this means, Alex,’” Vasarhelyi laughs. Adds Chin, Alex was like, ‘Oh yeah, that’s cool.’ But we were on the other side of it, after a big day of filming, changing diapers, and doing all that stuff.”

Meru made over $2.3 million at the domestic box office, an impressive total. But Free Solo has far surpassed that, earning almost $11 million to date and finishing as the number one documentary in theatrical release 10 of the last 12 weeks.

“The fact that people are responding to it is very, very meaningful to us,” Vasarhelyi affirms. “It’s wonderful. It’s amazing. We’re shocked because it wasn’t something that we ever thought about.”

The film made the Oscar Documentary shortlist announced earlier this week, keeping it in contention for an Academy Award nomination.

Vasarhelyi has a theory about why the film has resonated with so many.

“I think people are very satisfied by watching someone have this audacious dream, and actually achieving it,” she says, adding, “There are very few things of virtuosity or genius that you can actually witness. And with those types of stakes. And this, you could witness. You could be with him. You could try to imagine what it felt like, and also understand the very human person behind it.”