James Cameron explains why we had to wait so long for Avatar 2

People can't stop talking about the Avatar sequel following the release of its trailer today (2 November), and now James Cameron has revealed why the film took so long.

Yes, we feel old just reflecting on the fact that it's been 13 years.

Cameron explained that the biggest challenge in making the film came from the fact that it's set underwater, which, as you can imagine, made its creation a lot harder.

In fact, a specially built 900,000-gallon tank was required for filming - but the producers will be getting their money's worth as it's also being used for the third and fourth films in the franchise too.

Because of the sheer amount of water involved, it meant that a lot of precautions had to be taken to help the cast do their jobs, and for some, that meant becoming scuba-certified.

This gave them the ability to hold their breath under water for a minute and free dive - that is diving without the help of breathing apparatus.

Cameron proudly said that Sigourney Weaver, 72, trained herself to hold her breath for six and a half minutes because of the film, and Kate Winslet, a new cast member, 'blew everybody away when she did a seven-and-a-half-minute breath hold.'

However, while this is obviously impressive in and of itself, apparently there are even bigger plans on the horizon for future Avatar films.

Producer Jon Landau said: "What we are doing now, from a story standpoint and a world standpoint, is on a much larger scale.

"That's both exciting and challenging. We are putting much more detail, first and foremost, into the performances of the cast, but we're [also] putting much more detail and diversity into the world that we are creating."

Avatar: The Way of Water will be released on 16 December.