“It all started when I was about 11, growing up in Los Angeles,” recalls Robert Redford. “I had a mild case of polio—not enough to put me in an iron lung, but enough to keep me bedridden for weeks. As I came out of it, my mom wanted to do something for me. She realized that, growing up in the city, I’d missed out on a lot of nature. So she drove me to Yosemite. If you’re coming from Fresno, you go through a mile-long tunnel, and when we came out the other side, I was blown away. We stopped to look at the view, and when I went to the edge—well, I said to myself, ‘I don’t want to look at this. I want to be in this.’”

That was 1949, when the National Park Service was 33 years old. This year the NPS celebrates its centennial, and Redford will commemorate the occasion by narrating an Imax film, debuting on giant screens across the world on February 12. National Parks Adventure—told from the perspective of world-class mountaineer Conrad Anker and his friends—will wander from Yellowstone to the Everglades to the Redwoods. The shoot employed an aerial film crew to capture the swooping grandeur of the country’s wild areas set aside for posterity.

There’s no way for the movie to include all 409 national parks, but as Redford points out, even one is enough to open the door to a life in the outdoors. “A few years after that first visit, I applied for a job in Yosemite. I spent two summers working at Camp Curry and at Yosemite Lodge as a waiter. It gave me a chance to really be there every day—to hike up to Vernal Falls or Nevada Falls. It just took me really deep into it. Yosemite claimed me.”

Of course, Redford—who went on to preserve a gorgeous valley in the mountains above Park City, Utah, and who has been active in dozens of conservation campaigns—was not the first American to be claimed by that high granite Yosemite landscape. David Brower, who built the modern Sierra Club, was America’s great alpinist of the prewar years, pioneering dozens of routes from the valley floor (often in the company of Ansel Adams, whose still camera was the Imax of his day). Before them, there was John Muir, who in the late 1800s invented the grammar and vocabulary of wilderness during one ecstatic summer in the high Sierra. (Redford, founder of the Sundance Film Festival, is currently developing a movie about Muir for HBO, which he will direct.)

Like the great conservationists, Redford keeps an eye on the status of the parks. “Even in the ’40s and ’50s you could sense things were going to change. Development was increasing, tourism was increasing. And none of it is helped by the structure of Congress, all the partisan fighting. The right has such antiquated ideas—if they took charge, I think they’d want to close the parks, open the land up for development. It’s an ongoing battle to keep the parks strong.”

A battle, Redford insists, that’s well worth fighting: “It may be that those are the only places where new generations can see nature as it once was.” For many, he says, he hopes the Imax screen will be like the highway overlook of his boyhood: a way to make people want to be in that amazing landscape.

National Parks Adventure can be seen at Samuel C. Johnson IMAX Theater at the National Museum of Natural History in Washington, D.C. Purchase tickets here.