Merriam-Webster defines horror as “painful and intense fear, dread, or dismay.” The best films in this genre, like The Exorcist, The Blair Witch Project, and The Witch, display our worst fears and suggest that those dangers are very real. In this regard, Free Solo—the 2018 Oscar-winning picture that documents Alex Honnold’s historic, ropeless ascent of Yosemite’s El Capitan—is absolutely a horror flick, and it’s the scariest outdoor movie you can watch this Halloween.

If you haven’t had the pleasure (or misery) of seeing Free Solo, just listen to Tommy Caldwell’s synopsis. In the film, he boils down the complexities of Honnold’s feat into a simple analogy: “Imagine an Olympic gold-medal-level athletic achievement that, if you don’t get that gold medal, you’re going to die. That’s pretty much what free soloing El Cap is like. You have to do it perfectly.” Codirectors Jimmy Chin and Elizabeth Chai Vasarhelyi add another layer of dread by filming Honnold’s loved ones as they watch him risk falling to his death. The experience fits Merriam-Webster’s horror definition to a T.

The true horror of ‘Free Solo’ is not watching Honnold pull off 5.13a moves thousands of feet above the ground. It’s spectating those around him as they wrestle with that awful possibility in their heads, like a good psychological thriller.

Punch “scary outdoor films” into Google, and it will spit back titles like The Evil Dead (really?), Cabin in the Woods (a dark comedy), and Cabin Fever (also a comedy). It’s preposterous that Free Solo is not the first and only search result. In other horror pictures, we often know the hero will survive, but the more we are removed from that thought, the scarier it is. Free Solo succeeds in making us feel like Honnold is on the brink of falling, even though it’s not a mystery whether or not he will prevail. After one Outside staffer’s first viewing of Free Solo, she told me: “I kept telling myself over and over in my head that I knew the outcome—Alex was alive and well! He just posted on Instagram!—but the sweeping wide-lens shots of him clinging to El Cap made my brain refuse to listen to reason.”

But the true horror of Free Solo is not watching Honnold pull off 5.13a moves thousands of feet above the ground. It’s spectating those around him as they wrestle with that awful possibility in their heads, like a good psychological thriller.

“My fear is that I’ll be filming and I could potentially see Alex fall through the frame,” Chin says in the film. As audience members, we’re putting ourselves in the shoes of Chin and the supporting cast—not Honnold’s—as we try to digest what’s unfolding before us. But most of all, we’re viewing this horror show from the viewpoint of Sanni McCandless, Honnold’s girlfriend. Some argue that Free Solo isn’t so much about the act of climbing El Cap as it is about their relationship. They’re right: no one is forcing Honnold to climb, and McCandless has to reconcile that fact with his unstoppable desire to push himself. We, along with McCandless, ask: Why do you need to do this?

The whole crew had to deal with this gigantic mental struggle. In one scene, professional climber and cinematographer Mikey Schaefer films Honnold from the Yosemite Valley floor. As Honnold reaches pitch 27, Schaefer can’t bring himself to look through his camera lens any longer. With his hand covering his mouth, Schaefer has to turn away from the tripod. It’s as if he’s cringing as the protagonist in a horror flick approaches the house where the killer awaits. In this case, the protagonist is his close friend.

In a behind-the-scenes clip, Schaefer explained the source of his fear. “There were many times while shooting the actual free solo that I had a hard time watching. I think a lot of that was due to the fact that I know the route. I’ve free-climbed El Cap with ropes,” Schaefer said. “But I know those holds. I have grabbed those holds numerous times and know how slippery they are, I know how insecure that move is. In a way, it would be a lot easier if I didn’t know.”

It’s as if he’s cringing as the protagonist in a horror flick approaches the house where the killer awaits.

Schaefer’s explanation brings up a good point. Free Solo instills fear in two different types of audiences: those who don’t know much about climbing, and those who climb and know how insane and earth-shattering Honnold’s ascent is. Schaefer obviously falls into the latter category. But his knowledge is actually a curse, and that’s what makes Free Solo so edge-of-your-seat terrifying for climbers.

The same is true for nonclimbers (at least some). No one else in my family climbs, yet when I took them to see the film, I could hear their gasps, which were notably louder when Honnold did the “karate kick” move on the crux. Another Outside staffer, who is “decidedly not a climber,” told me: “The first time I saw Free Solo, I was sweaty the whole time and white-knuckle-gripping the armrests of my theater chair.”

I’ve seen Free Solo five times. My palms were sweaty during each viewing. And I’m certain they will be the next time, too—when I watch it again on Halloween.