The Alaskan environment itself trying to kill them.
That loud roar the Alpha lets out when Diaz was taunting him after killing one of the wolves. You can see the breath of the Alpha in the moonlight. Then it's joined by the breaths of all of his packmates.
The moment near the beginning when they realize how severe the wolf problem is. It's night, they hear something move in the distance, walk over with torches in hand and look into the darkness. They see a pair of wolf eyes glowing in the reflected light of Ottoway's torch and stand together to stare down the wolf. All of sudden, about SEVEN pairs of eyes come into view, not attacking them but just watching and waiting.
The scene where Talget was preparing to cross the ravine. An extended shot of him trying to unfog his glasses and psyche himself up, with no music playing. The scene is identical to previous scenes where a wolf suddenly jumped out and mauled the person to death. The tension could be cut with a knife.
Henrick's drowning scene. And all because of a stupid thing like a boot caught between some rocks. Made worse because [for one thing, he was screaming the entire time and for another, his head was literally just INCHES away from being able to breath above the surface.
Just after the plane crash, Ottway helps a man from being trapped in his seat...next to the severed lower half of the guy sitting beside him.
