Jennifer Aniston's No. 1 Fear!

In a candid interview with “Extra's” Renee Bargh, the 45-year-old star opened up about why she's developed a lifelong phobia of being underwater.

“It's not a love of mine to sink underwater,” she said, adding that a terrifying childhood moment triggered it.

“I drove my tricycle into a swimming pool when I was 5 and I didn't let go and I sunk to the bottom,” she said. “I was not a bright child and my brother dove in and got me out of there.”

Aniston says that incident made the underwater therapy scene for her new critically acclaimed role in “Cake,” extremely hard to film.

“I grabbed these weights you use for therapy and I'm supposed to sink to the bottom of the pool so I can see if I can have the guts to kill myself. It took me thirty some odd takes to get any take that you saw me going down.”

Renee asked, “Do you feel like you overcame that fear?” and Jen quickly shot back, “No. I will never do that again. Hardest thing in the world. Done.”

Facing her fear on film did pay off. Some critics called it a career-changing role for Jennifer, who stars as a woman struggling with drug addiction when she gets hooked on pain meds after a car accident.

Aniston, who is up for a Screen Actors Guild award on Sunday, is not bitter about being overlooked for an Oscar nomination.

She's thrilled with all the love her performance and the small indie film are getting right now. “It feels so special… I don't really care about all that stuff and then when it happens you go, 'Oh, this is actually really nice.'”