Leonardo DiCaprio almost drowned while shooting footage for pal Fisher Stevens' documentary about marine biologist Sylvia Earle in the Galapagos.
The movie star joined the filmmaker and mutual pal Edward Norton for the trip, during which DiCaprio was handed an underwater camera and told to capture shots of Earle diving for a short film Stevens was making.
But The Revenant star's oxygen tank was faulty and what started out as a pleasant swim almost cost him his life.
Stevens tells GQ, "I was filming Sylvia and I had this little easy camera to shoot underwater, and he was Sylvia's diving buddy, so I said, 'Would you film Sylvia?' And he said, 'Yeah, I love it, man, I love it'. I was diving buddies with Edward Norton.
"So we go down and we see 300 eagle rays and spotted rays and it was an amazing dive. Leo bolts away with Sylvia, and Edward goes in front of me, and the next thing I know after 20 minutes, I'd lost them all.
"Then, I see Leo buddy breathing, because Leo's tank was leaking oxygen, and Edward had to save him! It was pretty crazy. But he actually did get some film for me and it was good for a second and then it got pretty shaky when he couldn't breathe. But we really bonded on that trip."
The life-threatening dive didn't put Leonardo off from working with Fisher and the two have now teamed up to shoot environmental documentary Before The Flood, which premiered at the United Nations Headquarters in New York on Thursday (20Oct16).
The pair was joined by Mark Ruffalo, Norton and his wife Shauna Robertson, Lukas Haas, John Leguizamo and his wife Justine Maurer, and Secretary of State John Kerry.
The film follows DiCaprio around the world as he attempts to understand more about the environmental threat to the planet because of climate change.