One of the very first things we learned about The Avatar Sequels, before we even knew there would be four of them, was that we’d be going underwater on Pandora. Writer/director James Cameron has been teasing for years that the follow-up films will offer brand new visuals as he aims to set some of the story under water on the fictional planet. But capturing this footage proves difficult given that the Avatar films are mostly made using motion-capture technology, wherein actors wear suits with dots all over them that track their movements and expressions and are then processed in a computer to result in the digital performance. Doing this underwater is, well, tricky, and it at least partially explains why it’s taken so long for The Avatar Sequels to happen.

Collider’s own Christina Radish recently spoke with James Cameron in anticipation of the upcoming NatGeo special Titanic: 20 Years Later which premieres on November 26th. During the course of their conversation, with Cameron currently in production on Avatar 2 and 3, she asked about the process of shooting underwater motion-capture. As Cameron explains, it’s incredibly hard:

“Well, we’re doing it. It’s never been done before and it’s very tricky because our motion capture system, like most motion capture systems, is what they call optical base, meaning that it uses markers that are photographed with hundreds of cameras. The problem with water is not the underwater part, but the interface between the air and the water, which forms a moving mirror. That moving mirror reflects all the dots and markers, and it creates a bunch of false markers. It’s a little bit like a fighter plane dumping a bunch of chaff to confuse the radar system of a missile. It creates thousands of false targets, so we’ve had to figure out how to get around that problem, which we did. Basically, whenever you add water to any problem, it just gets ten times harder. So, we’ve thrown a lot of horsepower, innovation, imagination and new technology at the problem, and it’s taken us about a year and a half now to work out how we’re going to do it.”

It’s been so tough, in fact, that they only had their first successful run last week. But Cameron is happy with the result thus far:

“We’ve done a tremendous amount of testing, and we did it successfully, for the first time, just last Tuesday [November 14th]. We actually played an entire scene underwater with our young cast. We’ve got six teenagers and one seven-year-old, and they’re all playing a scene underwater. We’ve been training them for six months now, with how to hold their breath, and they’re all up in the two to four minute range. They’re all perfectly capable of acting underwater, very calmly while holding their breath. We’re not doing any of this on scuba. And we’re getting really good data, beautiful character motion and great facial performance capture. We’ve basically cracked the code.”

The filmmaker also adds that the most underwater-heavy entries in The Avatar Sequels will be the second and third installments, but there will still be some underwater elements in the final two movies: