James Cameron's sequel to 'Avatar' will be "monumental".

The director pioneered filming in 3D with the film, about the blue Na'vi - alien inhabitants of another planet - and Sam Worthington, who starred in the movie, says he is to try something even more adventurous for his next film, possibly setting it underwater.

He said: "I spoke to James recently and he told me the story and it's monumental, where he's thinking of taking it. It's just a matter of him writing The Script, piecing it together and figuring out how to push himself. He wants a challenge. And he wants to improve the audience's experience. He's been messing about with 3-D and frame rates a lot more."

There are rumours that the cast could go on a trip to experience the rainforest in preparation for the film, and Sam is prepared to go anywhere James wants in order to challenge him.

He added: "I'm up for whatever James wants me to do. If he wanted me to go to the moon, I'd do it. He's the type of guy that, not just as a director, but as a good friend, challenges me as a man.

"James went on a submarine for his birthday and he's been doing a lot of free diving. I happened to be working when he went underwater, but I'd have loved to go."