Featuring Howard and Michele Hall, Rusi Vulakoro, and Jean-Michel Cousteau. Unrated.

Although the aim of IMAX films is to entertain--all those tiny bums in popcorn-littered seats--there is also something implicitly political in these megascreen depictions of our rapidly defoliating planet. Whether tracking endangered pandas or exploring the ice caves of Antarctica, these 40-minute movies are quiet provocations for those wee ones to do something about human-made dangers, and before their bums get too big (from all that popcorn, for instance).

Coral Reef Adventure is the only such item I can recall that ends with a direct rebuff to George W. Bush and his "It takes a pillage" cronies, although this comes in the polite circumlocution of asking viewers to "support leaders who oppose global warming". The preceding 39 minutes, although elegantly assembled, can just about get you mad enough to figure out what that phrase means.

The film follows a husband-and-wife team of veteran divers, Howard and Michele Hall (who have directed a couple of IMAX features themselves), on their visits to threatened reefs around the Pacific. Their journey is prompted, dramatically speaking, by complaints from Fijian marine biologist (and perpetual guitar player) Rusi Vulakoro, who discovers that the coral off his tiny island is rapidly deteriorating. Indeed, when the Halls get past a small protected area bristling with lushly coloured life, they find a large expanse of once-thriving reef now suddenly as white and wrinkled as Dick Cheney's ass.

After considerable investigation--involving, of course, some stomach-churning aerial footage--our heroes figure out where the changes are coming from. Fiji, like many ocean-bound places, is suffering from a trifecta of ill effects: global warming, with even a two-degree temperature shift bumping off necessary organisms; overfishing, which removes large predators that protect coral from midsize critters; and overlogging, which sends coral-choking riverbed silt into coastal waters.

Overall, though, the film emphasizes wonders over horrors. Chief among these are the many Finding Nemoí‚ ­like scenes of symbiotic interspecies relationships. You have to love the shrimp and the fantail fish working together on building a house (to the strains of Crosby, Stills, and Nash). And there are some (stagey) chills, as when Howard gets hauled by a wild undercurrent into the path of hundreds of grey reef sharks. Most memorable, perhaps, is the long scene in which an Australian scientist opens her mouth to let small crustaceans clean her teeth: a rare example, perhaps, of us letting them do the eating.