Kate Winslet spreads her fins in underwater set photo from 'Avatar 2'

Kate Winslet's role in Avatar 2 remains something of a mystery, but now we've got a glimpse of her sub-aquatic stunt work with a new on-set picture.

The Titanic star is seen on the movie's underwater set, on which she showed off her newly-acquired training in free diving.

With the underwater camera crew to her right, all hooked up to oxygen, Winslet is at the bottom of the pool, arms spread wide with just a nose clip.

Watch: Signourney Weaver refused a diving double on Avatar 2

As the picture, from the official Avatar twitter feed, notes, she learned how to hold her breath for minutes on end for the shoot.

Read more: James Cameron confirms end of Avatar 2 shoot

She told The Hollywood Reporter over the summer: “I had to learn how to free-dive to play that role in Avatar, and that was just incredible. My longest breath hold was seven minutes and 14 seconds, like crazy, crazy stuff.”

In the movie, she plays Ronal, a free-diver of the Metkayina tribe on the planet of Pandora, a clan of 'reef people'.

Winslet is not the only one to be trained in the art of free-diving for the long-awaited sequel from director Jame Cameron.

Sigourney Weaver, who is returning as Dr. Grace Augustine, said in an interview with The New York Times that she was also able to hold her breath underwater for extended periods of time, and that with the help of elite military divers, she could do so for over six minutes.

Pictures released in September also revealed Weaver, who refused the offer of a diving stunt double, in underwater mo-cop scenes.

Read more: Weaver joins Justin Long on set of Avatar sequels

It follows a number of recent shots from the film's set, including the first of Sopranos star Edie Falco's General Ardmore, of the occupying Resources Development Administration.

Winslet and Falco, along with the likes of Oona Chaplin, Michelle Yeoh and Jermaine Clement, join with returning stars Sam Worthington, Zoe Saldana, Giovanni Ribisi and Stephen Lang on the long-awaited, long-delayed movie.

It's currently due for release on 16 December, 2022.

Watch: James Cameron updates on Avatar sequels