But Gyllenhaal admits there's another reason behind the icy excursion.

He tells the New York Daily News, "The only reason I'm doing it is I wanted to do something that absolutely terrified me - killer whales and walruses and freezing cold water? That seems absolutely terrifying."

And the actor insists he's not worried about reaching the landmark age after a chat with his sister Maggie Gyllenhaal.

He says, "I was talking to my sister the other day about being 30 and celebrating the life you've had up to this point. I feel there's a massive sort of sea change coming in myself and I'm psyched."