Special Note: "Sanctum" is being touted for its theatrical release as having utilized the same 3-D cameras and process for its filming that producer James Cameron used for 2009's "Avatar." That is all well and good, but does this particular movie excel with, or even warrant, a so-called added dimension? In short, no. In long, "Sanctum" is a naturally drab film (it's set in a cave, after all), at times verging on monochromatic in its color scheme. The 3-D frequently is engulfed by the darkness of the story's setting and does not lend itself positively to the format. Watching the film, I kept asking myself (1) if the 3-D was necessary, and (2) if it would lose any of its power if shown in regular 2-D. It is not, and no, it wouldn't, leaving the 3-D presentation utterly useless.

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Can a motion picture overcome a lousy script if it is an otherwise effective, technically proficient piece of filmmaking? "Sanctum" sure-footedly answers this question in the affirmative. Loosely inspired by true events that screenwriter Andrew Wight personally endured, his and co-penner John Garvin's ignorance at tackling a script (this is their first) is given away at every turn. The characters are two-dimensional types, the relationships they attempt to build are excessively over-the-top, and the dialogue is often so on-the-nose and made up of so many clichés that it causes bad laughs. When they finally shut up and Australian director Alister Grierson gets down to business, however, "Sanctum" works like gangbusters. Consistently stirring and suspenseful, the core conflict of a cave expedition gone horribly wrong is ripe for a tension-filled cinematic telling. On these grounds, the film does not disappoint.

Papua New Guinea's Esa-ala, said to be the largest unexplored cave system in the world, is where no-nonsense professional cave diver Frank McGuire (Richard Roxburgh) and his team have been based for the last thirty-four days. Overextended and tired, they are joined on the last leg of their research by Frank's sparring 17-year-old son Josh (Rhys Wakefield), wealthy entrepreneur Carl Hurley (Ioan Gruffudd), and Carl's inexperienced girlfriend Victoria (Alice Parkinson) just in time for a bad storm to rustle up and a falling boulder to block their exit. With the water quickly rising, the group have no choice but to move forward in hopes of finding a different way out and reaching the surface. As the terrain grows increasingly perilous and their oxygen tanks and lights dwindle, it isn't long before panic, hypothermia, illness, bodily harm and paranoia set in. Amidst it all, Frank and Josh are finally forced to face their troubled father-son relationship head-on. When all is said and done, there might not be a "later."

"Sanctum" does not open with promise, the dialogue foreshadowing what is to occur with such obviousness one almost cannot believe their ears. "What could possibly go wrong?" Victoria says before she repels herself into the mouth of the cave. "I'm gonna marry that girl," Carl later utters, sealing her imminent tragic fate. At another point, Carl transforms into a morale-boosting Hallmark card and tells Josh, "Life is not a dress rehearsal. You have to seize the day." If all that weren't misguided enough, Frank's and Josh's animosity is set up by characters coming right out and saying things like, "Your father is really mad at you," while a few minutes afterwards Frank hollers at Josh himself, "I'm really angry with you!" Their relationship doesn't get much more subtle from there—Frank comes off as a hot-headed brute, while Josh is insufferably whiny and unreasonable—though they do, late in the film, repair some of this damage with a few heartfelt exchanges.

So, yes, the writing isn't the movie's strong suit. Fortunately, the high-stakes fight for their lives that the ensemble find themselves in is involving enough to level things off. Lushly photographed by Jules O'Loughlin and seemingly shot in real locations rather than on manufactured sets, there is an authenticity to the proceedings that only adds to the viewer's claustrophobic apprehension. The R rating is also a welcome surprise, not because there is an overabundance of violence (there isn't, save for one grisly scene involving a person's hair getting caught on their equipment as they perilously dangle in midair), but because the language used and brutality of the situations portrayed feel honest. For once, a filmmaker's vision has not been compromised simply to draw in a bigger audience with a kid-friendly PG-13 rating. Director Alister Grierson, taking a page from the skillfully orchestrated anxiety brought to 2006's "The Descent," earns a lot of mileage out of expertly milking the riveting, at times unbearably taut predicaments his characters face; a stunning underwater set-piece where a diver is left to drown after their oxygen is compromised is especially frightening.

Where "Sanctum" ultimately leads is either telegraphed in advance or just largely predictable, but it is the journey to this point that should please anyone seeking an adrenaline rush. There are plenty of action-adventure pics released each year, but so few of them are able to generate real momentum and excitement that when one does, it's worth making a racket about. "Sanctum" is, perhaps, too wobbly in its foundation to be considered a great film—the writing is the worst offender, followed by a grating performance from newcomer Rhys Wakefield, as the holier-than-thou Josh, that never matches the restrained intensity of veteran Richard Roxburgh (2001's "Moulin Rouge"), as father Frank—but it is a well-crafted, impressively executed one. Once it's over, the mere thought of ever stepping foot in a cave again will be out of the question.