Thor, is that you?

Chris Hemsworth -- who's known for looking good shirtless -- snapped a selfie on Sunday that revealed just how frail-looking he became for his In the Heart of the Sea role.

"Just tried a new diet/training program called Lost At Sea," he Instagrammed to his 869, 000 followers, "Wouldn't recommend it.. #IntheHeartoftheSea"

Ron Howard's action-adventure movie follows the aftermath of the real-life maritime disaster that inspired Herman Melville's Moby Dick. In the winter of 1820, the New England whaling ship Essex was attacked by a whale, leaving the surviving crew to do the unthinkable to stay alive in the face of storms, starvation, panic and despair.

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Hemsworth stars as the vessel's veteran first mate Owen Chase, and revealed to ET that he had to drop a good amount of weight to shoot the shipwrecked scenes.

"We have to shoot the really skinny stuff where we drop down to five, six, 700 calories a day, a good three or four weeks and it's going to be pretty uncomfortable, but we'll be together in our misery," the 32-year-old actor said. "I spend more time thinking about food than anything else at the moment."

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In the Heart of the Sea hits theaters Dec. 11.

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