I'm not certain how ashamed I'm meant to be that I didn't find Sanctum to be at all bad. Mind you, I didn't find it particularly good, but it feels like the sort of movie one should hate with a full-throated hate. Thing is, the film only really sets out to do one thing, and it does it really damn well.

Chiefly, the film serves as a tech demo for the next generation of James Cameron's custom-made Fusion Camera System: Avatar was so 13 months ago, doncha know, and bleeding-edge tech fetishist needs to keep himself in the game somehow. Oh, it's not "his" film: it was co-written by Andrew Wight (very loosely basing the story on an event that he personally experienced) and John Garvin, who I take to be an extreme sportsman (he's also credited as the film's diving coordinator), and directed by Alister Grierson, an Australian filmmaker hand-picked by Cameron during the Avatar shoot. Cameron is credited merely as producer (and apparently, as a corporation; the opening credits begin "JAMES CAMERON / UNIVERSAL PICTURES / WAYFARE ENTERTAINMENT present", just like that), and some would theorise that he "produced" Sanctum rather like Steven Spielberg allegedly "produced" Poltergeist, but I'm not having it: if Cameron was indeed calling the shots, then he just made the worst film of his career since Piranha II: The Spawning, and that seems kind of unlikely to me.

That said, if there's a concept that absolutely bellows "This is a James Cameron project", it's 3-D underwater cave photography. Which is, to tie everything back together, the one thing that Sanctum really cares about doing, and the one thing it does really damn well. No accident that Wight, prior to making his unpremeditated jump into feature screenwriting, served as producer of the Cameron-directed 3-D underwater documentaries Ghosts of the Abyss (about the Titanic) and Aliens of the Deep (about the Abyss). The whole thing feels unmistakably exercise-ey, and if you wanted my guess - not that anybody asked for it, but still - I'd say Cameron threw the whole project together to see what kind of bugs would happen during an underwater Fusion 3-D shoot, prior to his rumored-to-be-underwater Avatar 2 going into pre-production. And this exercise would be shot in real and synthetic caves, because underwater 3-D cave photography is hella fucking awesome, and if you disagree than you hopelessly fail to understand what is awesome.

Having thus set the wheels in motion, Cameron skipped off to do whatever it is that James Cameron does for fun - vaporising solid gold bars and huffing the fumes, I imagine - leaving Sanctum in the hands of two first-time writers and a low-budget indie director. They got the job done, at least: like the Rocky Mountains or the New Mexico desert, underwater caves are simply too inherently wonderful to look at for even the most gruelingly incompetent filmmaker to bungle it. Which isn't to say that Grierson and cinematographer Jules O'Loughlin couldn't possibly have done a better job, of course, but the raw spectacle inherent in some of the most uncommon geography on earth captured in the best 3-D money can buy counts for something. Through God help the poor soul who sees the film in 2-D, whereupon it would have absolutely not a single merit. Say what you will about the technology but as a completely shallow vehicle for deploying spectacle, it is well-matched with a film that is also a completely shallow vehicle for deploying spectacle. But spectacle has its earned place in the world of cinema as surely as anything else does. There's not much use to cinema if it can't transport us, I like to say, and Sanctum transports, in those wonderful moments when nothing is happening except for people paddling around.

Those moments are not everything, unfortunately; like Cameron and Avatar before them, the Sanctum filmmakers unfortunately don't have the simple human decency to make an environmental tone poem, but tart their wonderful, wonderful display of beautiful places and languidly diffuse lighting up with a goddamn plot. And boy howdy, does Sanctum have a real doozy. In short, there's a young man - how young is sort of fuzzy - named Josh (Rhys Wakefield), who hates his dad Frank (Richard Roxburgh), the world's greatest cave diver. It's in that capacity that Frank has been hired by Carl (Ioan Gruffudd), an billionaire American with a zest for extreme adventure; he's arranged to lead an expedition into the world's largest uncharted cave system (which seems, on the face of it, a logically unsound claim), deep in the Papua New Guinea jungle, here played by Australia.

Skip ahead, those three plus some other people who don't particularly matter, they all get trapped way deep down inside the cave when it starts to flood during a massive storm (the entrance is a sheer vertical well some tens of feet across, all the better for catching rainwater). With nowhere to go but into the submerged portions of the cave system, our clutch of disposable characters make stupid mistakes and die one at a time, slasher-style, though the film's obvious debt is owed rather to the disaster films typified by the work of producer Irwin Allen in the 1970s (the sheer quantity of water particularly reminded me of The Poseidon Adventure), especially given the incredibly expected way that Josh and Frank patch their torn relationship as a direct result of their shared danger.

The twin sins of Sanctum as a narrative film are the barbaric familiarity of its situations, and the profound emptiness of its characters, the latter of which is made far worse thanks to some bargain-basement performance. Early on, before his character (like all the others) is largely reduced to screaming, I wondered if Gruffudd, barely submerging his native Welsh accent in the worst American accent to grace screens in an age, was on some kind of stimulant, so flighty and shrill was his performance; and then to pair that with Wakefield, so inanimate, so painfully inarticulate, as though the mere act of speaking hurt his entire body... I was irresistibly reminded of the collision of Jeremy Irons's arch-camp villain and Thora Birch's catatonic princess in the incomparably bad Dungeons & Dragons, which is not a movie you ever want to think about when you're watching any other movie. Only Roxburgh, underplaying his character as all gruff wisdom and spiky impatience, actually gives a good performance (even a very good performance, I'd say, but the script holds him back), completely unrecognisable in every way if, like me, your chief context for the actor is the reptilian Duke in Moulin Rouge! All in all, it's boring more than despicably bad, but some of the dialogue - which in the second half of the film largely consists of variations of "Oh shit!" "Don't do that!" "Oh fuck!" "Come do this!" "Oh fucking shit!" "Do what I say or we'll all die!" - ought to win some kind of prize for its ghastly functionality, as does the shameless use of Coleridge's "Kubla Khan" as thematic corsage. And the sheer joylessness with which the film doles out character deaths is, if not misanthropic, at least crudely amoral.

Basically, the film breaks down like this:

-It is excellent during the underwater sequences when everybody just shuts up and explores.

-It is fairly good during two of the OMG! Tension! scenes, through some slightly erratic though happily non-hyper editing (I suspect the coverage was shot from lousy angles).

-It is impressive, but weakly so during the scenes introducing us to the undeniably photogenic cave entrance; the one place the CGI looks like CGI. Also, for a reason I cannot begin to explain, all of the exterior 3-D cinematography looks ineffably "off", a little more jarring and artificial than in the best of the form.

-Any time that any human being does or says anything, it completely sucks.

You know what? I liked it. I'm glad I saw it. It's not a good "movie", but it's a fun kaleidoscope that only just starts to feel a bit too long at a slice under two hours. I would never recommend it to another soul. Nor would I aim to talk somebody out of it who wanted to see it anyway, and was willing to put up with some grating talking scenes to get to the money shots. Because, Christ, underwater caves in 3-D.