Knowing the outcome doesn't lessen the grip of the storytelling -- individual moments are telling, lesser-known tragedies dot the desperate rush to act, and at each point, the jagged tunnels and torrid currents exert an almost tactile hold.

[A] hugely involving documentary...

The film-makers somehow manage to wring nerve-jangling tension from a foregone conclusion.

Despite its mountainous minutiae, the film never feels anything close to overlength or overkill, juxtaposing real life footage with the manufactured in the service of human possibility, perseverance and truth (huge credit to editor Bob Eisandhardt).