James Cameron's latest adventure finds him underwater again, this time in 3D cave-diving adventure 'Sanctum.' Based on actual events experienced by co-producer Andrew Wight, the film follows a group of divers trapped underground by a freak storm. Full of stunning visuals, 'Sanctum' is a movie for divers, by divers. (Random fact: James Cameron is an avid diver.)

Moviefone spoke with Karl Shreeves, the Technical Development Executive for the Professional Association of Diving Instructors (PADI), who's very familiar with all aspects of diving and diving instruments. Shreeves spoke with us about 'Sanctum,' monsters of the deep and what to really be afraid of down there.

Moviefone: Are there a lot of things to be afraid of when you go cave diving?
Karl Shreeves: Diving is like a lot of sports and activities. There are different levels of participation. If somebody is just starting out, you're not going to go into a cave. What you see in 'Sanctum' is the most extreme, out-there kind of diving, and then they have a 'perfect storm' of errors and conditions, stuff that is not very likely to happen in real life.

Caves present their own challenges, then?
As an experienced diver, you need to be trained extensively before you go cave diving; when you're down there, you don't have the ability to come straight back up to the surface. Caves are beautiful, but if you don't have a light, there is zero light. There are passages where you can get lost, and you can stir up the bottom and not be able to see inches in front of your face. And, of course, you're carrying your life support on your back, so you have to plan that you have enough air to get in and out –- not to mention some back-up for problems that might occur.

Experienced cave divers hardly ever have any problems when they're in caves. When you read the statistics about accidents, they're usually involving people who shouldn't have been down there in the first place. Accidents when cave diving are the result of bad decisions. People make bad choices, which leads to bad outcomes.

In the movie, are they experts or amateurs?
They're experts, and they're in a combination dry-and-underwater cave. It's not as common. Most cave diving is underwater, but what they show does happen. The actual event that the story was inspired by did actually happen, too.

How accurate is the depiction of cave diving in 'Sanctum'?
The movie is done with very good technical accuracy. I'm acquainted with Andrew Wight, who's also a diver. In the movie, you see the actors wearing rebreathers and other equipment –- and they're not Hollywood props. They're the real thing. It's about as close to cave diving as you'll get without actually doing it.

The only exception is the air pockets the divers miraculously find. In most cases, in that type of cave, there is no air. You can't just swim up and expect to find a pocket of air for you to breathe. But then again, this is a movie, and where would the dialogue take place if not for the air pockets?

How big a danger does 'the bends' (decompression sickness) pose?
They have that in the movie too –- and they even depict that well! It's a risk, but it's a very remote risk. There are millions and millions of dives every year, and very few cases of DS are reported. And of those cases, most of them are divers who failed to follow procedure.

How often does the weather affect a dive?
It's very rare. In fact, the only time I can think of when it really happened is the actual event that inspired Andrew Wight to write the story. They were exploring caves in the Australian desert, and a freak storm came and collapsed the cave with half the team still down there.

Forgive my ignorance, but are there any animals or creatures down in those caves?
Actually, there are. A lot of the diving I do is for science, so often I see things down there. There aren't any big monsters or creatures like in a horror movie, but you're talking about invertebrates, blind cavefish and occasionally some eels. The scenery and formations can be absolutely stunning.

How is the 3D in the movie?
One philosophy that both Cameron and Wight have had with 3D is that it should be like stereo music. That's how they did it. They didn't shove it in your face, but they set up good shots where the 3D adds a lot. If it wasn't in 3D I think it would have lost something, I can say that.

Have you ever seen a movie with a diving sequence that is just ridiculous to you, in terms of its inaccuracy?
[Laughs] Yeah, I hate to pick on anyone, but even some very good ones aren't very accurate. 'The Big Blue,' which is an amazing movie, has this thing where the diving team can't go deeper than 300 feet. They do this whole mumbo-jumbo about what will happen if you do, like you'll conk out and die. They needed a plot device to draw a line. 299, I'm OK! 301, I'm dead.

Unfortunately, bad depictions are common, so that's why I really admire 'Sanctum.' If you go back to 'Abyss' [also directed by James Cameron], you'll see that it was done very credibly. All the little sub-things, like Michael Biehn's character going psychotic because of the high-pressure nervous syndrome, that has actually happened in experimental dives. People react to helium. And the liquid-breathing rat, that was real too!

I think the viewer knows nonsense when he/she sees it, and you don't have to be a diver to know the difference between credible and non-credible.