This is rather like a slow-dawning version of the buried-alive situation Rex finds himself in at the end of The Vanishing (Spoorloos) - just as primal and fucked, but deceptively panoramic.
Even so it takes this pair of unlucky divers a remarkably long time to panic. Seven hours pass before they decide that it might be a good idea to drop their weights. "This can't be normal", observes Susan some time after that as dorsal fins start to break the surface around them.
When they do realise that they are dealing with something a lot more unpropitious than mere grounds for litigation, they talk themselves into a domestic which sounds a note of black humour.
The director Chris Kentis, himself an avid scuba-diver, has opted for the detached yet ultra-realist Dogme-style film-making. It cost less to make than half of the budget of a typical Hollywood blockbuster's sound effects budget.
The subject matter is based on "real events", the 1998 disappearance off Queensland of Tom and Eileen Lonergan, the result of a run-of-the-mill kind of cock-up on their dive boat. (Bit of a spoiler that link - sorry!)
Kentis deserves credit for the way that the pacing, structure and characterisation have been treated in light of the outcome.
Guaranteed to leave you feeling chilled for some time after viewing - and more deeply than standard-format 'horror' movies like Jaws or Blair Witch Project with which it has been compared.
Ebert puts it very well: "The movie is about what a slender thread supports our conviction that our lives have importance and make sense. We need that conviction in order to live at all, and when it is irreversibly taken away from us, what a terrible fate to be left alive to know it."