Beautifully back-lit bodies are suspended in water and suspended from the 3D screen in “Sanctum,” the new trapped-in-a-cave thriller that James Cameron produced as a way of showing off his love of all things underwater. It’s a solid, old-fashioned action yarn filled with the very latest dive gear and the oldest plot formula in the movie-maker’s playbook.

Set deep underground in the Esa Ala “caving district” of Papua New Guinea, it’s a “Ten Little Indians” story of a handful of explorers — some experienced, some novices — trapped underground when a cyclone floods “the largest unexplored cave system in the world.”

There are two central conflicts. Team leader Frank (Aussie actor Richard Roxburgh, in full growl) is the hardened veteran cave explorer whose son Josh (Aussie newcomer Rhys Wakefield) is in full rebellion against dad’s chosen profession and his place in it.

“It’s a sphincter of rocks,” he pouts. “NOBODY cares!”

Carl (Ioan Gruffudd) cares. He’s underwriting this expedition, ready to slap his name on places that “since the beginning of time, no human being has ever seen.” He’s readier to take risks than Frank is, and that’s the second major conflict set up here.

Carl dragged his mountaineering girlfriend (Alice Parkinson) down the huge hole with him. Then there’s Frank’s old diving mate, George (Dan Wyllie), an expert on “the bends” — “Your blood’s gonna fizz up like a dropped can’o beer.” And also along, Frank’s longtime aboriginal colleague, Luko (Cramer Cain).

In the first act, we see how even when all the hi-tech is working something terrible can go wrong when you’re squeezing through underwater tunnels with bulky dive gear on. Director Alistir Grierson has seen Cameron’s earliest underwater thriller, “The Abyss.” He knows there’s nothing more traumatic than watching someone panic and suffocate right in front of you. The 3-D makes that first death all the more traumatic.

Then the rains come and those five other folks are trapped below with only Frank, “the most determined cave diver in the world,” to get them out. Frank’s leadership consists of barking “You CAN do this because you HAVE to do this” and “This cave’ll kill you in a heartbeat.”

In true “Ten Little Indians” (an Agatha Christie story that set this formula in stone) fashion, the object of “Sanctum” is to make us wonder: Who will live and who will die next?

The movie, inspired by a real-life experience of Australian co-writer, explorer and Cameron consultant-pal Andrew Wight, is dully predictable, with touching moments of noble sacrifice and cliched scenes of rank cowardice. But the filmmakers make its stock-in-trade situations visually arresting.

Lacking supernatural elements, sex and over-the-top gore, “Sanctum” would have played better as a PG-13 picture. Gratuitous profanity doesn’t add edge to the Cameronesque tough-guy dialogue. And since you can figure out where it’s going long before our divers do, it seems too long.

But “Sanctum” does take us somewhere alien, if not exactly as dazzling as Pandora. And as a movie, it performs according to spec, allowing its producer to further perfect the gear and techniques that he is almost certain to turn lose on “Avatar 2” and “3,” somewhere down the watery road.

Film focus

WHAT: “Sanctum”

RATED: R for language, some violence and disturbing images

RATING: 2 ½ stars

RUNNING TIME: 1 hour, 49 minutes

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