Conceived in 3-D

If you think your boss is intimidating, imagine how Alister Grierson feels.

The Australian filmmaker, who made his new movie, “Sanctum,” under the guiding hand of “Avatar” creator James Cameron, had to present a finished cut to the famously exacting director in Cameron’s Malibu home theater.

“Every time he’d twitch, I wondered, ‘Oh no, what did I do wrong?’ ” says Grierson, 41, who had previously directed only one other movie, a small Australian war picture.

“It was like sitting with God at the pearly gates watching your entire life.”

Life and death are not quite at stake when “Sanctum,” a modestly budgeted ($30 million), 3-D, cave-diving adventure comes out in theaters today.

But the movie is the lowest-budgeted feature that Cameron, who serves as its executive producer, has been involved with in more than 25 years. And it’s an important test of his belief that 3-D can be as effective in an intimate, emotional story as it is in a grand epic.

The film tells the story of veteran cave diver Frank (Richard Roxburgh) and his alienated son Josh (Rhys Wakefield) who wind up, along with a larger diving team, trapped far below the surface when a freak storm hits during an expedition.

As the dangers mount and claim the lives of many on the team – think “Ten Little Indians,” only with natural disaster as the culprit – Frank and Josh must work through their issues if they are to survive.

The story and the emotional dynamic, however, can sometimes seem secondary to the filmmakers’ desire to show breathtaking underwater shots, such as characters navigating a claustrophobic tunnel or floating ethereally as though in a giant human aquarium.

While there have been a handful of low-budget 3-D films, such as last summer’s “Piranha 3D,” most have been after-the-fact conversions designed to take advantage of the higher ticket prices the format commands.

“Sanctum,” on the other hand, was conceived from the start in 3-D, and shot to take full creative effect of the tool.

It came together when Andrew Wight, a longtime Cameron friend and collaborator, decided to make a movie about a group of trapped cave divers using the same tools and techniques they deployed on Cameron’s documentaries.

Cameron agreed to godfather the film, Wight enlisted Grierson, and the team decided to shoot the movie in Australia in late 2009 and early 2010, using some of the same cameras that Cameron used on the groundbreaking “Avatar.”

While the trailer flashes his name early and often, Cameron plays down his involvement.

“I can’t stop the studio marketing people from picking the path that they think is going to make the most money for the movie, but I would hope people would look past that,” he says. “It’s Andrew and Alister’s film.”

Still, even casual movie fans will recognize many of Cameron’s visual signatures in “Sanctum” – a helicopter descending into an exotic landscape as in “Avatar” and “Aliens,” underwater action sequences a la “The Abyss,” and remote cameras of the type used in “Titanic” and his documentaries.

While Cameron came to the set only once – he was indeed busy making “Avatar” during much of the movie’s production – he helped develop the script and offered feedback in the editing room.

Cameron says that in addition to deploying his beloved gadgetry, “Sanctum” allowed him the chance to play with something else: a father-son relationship.

“I haven’t done (that) before,” he says. “And as the father of two boys, I thought this could be interesting to explore.”

For Wight, the story is even more personal. In 1988, while the producer was leading a cave dive in Australia, a storm like the one in the film trapped him and 14 others. The group survived, but only after two uncertain days spent trying to find a new way out.

“I reflected on the personalities and the people and how they got together in that survival situation,” Wight says. “And I thought, ‘Boy, there’s a fantastic film to be made out of that.’ ”

Cameron, too is a diving enthusiast. Wight has helped him go on half a dozen dives exploring various wrecks and deep-marine life while the pair filmed documentaries.

But “Sanctum” is not so much a labor of love as it is a labor of purpose, a chance to show the world that 3-D can be done well even at a smaller price.

“I wanted to demonstrate to the greater filmmaking community that you don’t need to be making a $300 million film to do a world-class 3-D film,” Cameron said.

“I was so tired of these stupid conversions being made and the idea being that 3-D is too expensive. You can make a world-class 3-D experience on a modest budget.”

Not that it was easy to do. Underwater movies tend to involve big set pieces, and big set pieces don’t come cheap.

To save money, Grierson was forced into some “MacGyver”-type solutions. The same set-built waterfall, for instance, was used for various locations in the cave, but shot so as to appear new each time.

“We used a lot of paint jobs and remodeling,” Grierson said.

The filmmakers believe the movie turned out to be a, well, watershed moment.

“I’d like to think there may be a point in time that people will look back (on ‘Sanctum’) and say, ‘Hey, that film was significant in that it showed people there was a way forward for 3-D,’ ” Wight says.

“3-D is part of the next evolution of cinema whether people want to admit to it or not.”