Sanctum (R) ★★½

What do you call a bunch of Australians tossed down a hole? A good start. I kid, of course – "a mediocre movie" is more like it. And that's precisely what you get with Alister Grierson's Sanctum, a 3D thriller in which a crew of cave divers struggle to survive after a monsoon-driven flood pins them thousands of feet underground.

Sanctum is set in Papua New Guinea but was mostly shot in the sprawling caves of South Australia. The cast is dominated by local actors, many of whom will prove unrecognizable to moviegoers residing above the equator – which, frankly, isn't all that much of a hindrance, since the lot of them will be killed off long before the closing credits roll.

The cast's lone non-Aussie – and the film's most familiar face – is Welshman Ioan Gruffudd, who plays Carl, a gratingly cocky American industrialist whose wealth funds the whole caving (the word "spelunking" is never used, much to my chagrin) expedition, and whose extreme-tourist bent compels him to come along for the ride. He also brings his girlfriend, Victoria (Alice Parkinson), whose strong-mindedness you just know is going to become a liability when the sh*t hits the fan.

The sh*t, in the case of Sanctum, is an apocalyptic storm that arrives days before it's supposed to, triggering an avalanche of boulders that effectively seals off all possible exits. With the water level rising and a near-zero chance of rescue, the group's hardened, no-nonsense leader, Frank (Richard Roxburgh), decrees that their best hope of survival lies in finding an alternate means of escape via an unexplored stretch of tunnels thought to lead to the ocean.

The situation grows gradually more desperate, and characters succumb, one by one, to the hazards of the deep in fairly predictable disaster-flick order. (The aging female is first to go, followed by the ethnic guy, etc.) Sanctum cycles through a series of grisly fatalities – including one delightful bit in which a shock of hair caught in a climbing apparatus results in an impromptu scalping – until finally the last man standing is Frank's son, Josh (Rhys Wakefield), a moody 17-year-old who has heretofore spent most of the film acting out with childish spite toward his neglectful dad. Out of supplies, exhausted, but with his exquisite surfer-dude haircut thankfully still intact, Josh must complete the remainder of the harrowing journey alone.

Director Grierson packs Sanctum with some truly breathtaking visuals. The underwater cinematography, shot with 3D cameras Grierson spent six-plus years developing, is particularly stunning. But the film's script clearly didn't receive as much care and attention as its cameras. The action is occasionally gripping, but the story lacks suspense, and its tone largely fails to evoke the gnawing claustrophobia that presumably festers in such a dark, musty subterranean labyrinth. Moreover, it's littered with truly execrable dialogue, made worse by ADR that sounds as if it were recorded in a cozy basement studio.

Executive producer James Cameron is featured prominently in Sanctum's advertising campaign, but the film itself bears scant evidence of his involvement, save perhaps for the splendid underwater scenes. I half-suspect he viewed the project as a tool to develop and test his 3D technology in preparation for his amphibious Avatar sequel. He certainly didn't use it to brush up on his storytelling skills.

Hollywood.com rated this film 2 1/2 stars.