A run-down of what happened (spoilers)
I enjoyed the movie quite a bit, even though it had it's weak point. It seems, however, that a lot of people didn't understand what was shown, and are left with questions. IMO, it all ties up neatly and it makes sense. Here's a run-down of what I think happens throughout the story.
******* Spoilers below *******
A team of drillers has been sent to the Northern Arctic to move in equipment, in order to continue a drilling project that was stopped in 1986. What's left of the project is a drilling site covered with a valve, wrapped in a box. Along with the drillers, the company has sent environmental experts to monitor that there are no negative environmental impacts; however, we find out that those experts are there more as a part of and attempt to appease the public, and aren't given any say in what's going on.
One of the environmentals (Hoffman) observes that the temperature is rising at an exponential rate. He makes a theory that this is melting the permafrost, releasing the CO2 trapped under it, which in turn is raising the temperature even more, creating a positive feedback effect, accelerating the process. He also theorizes that this might release "Sour gas" (H2S, i believe) which is trapped deep under the surface, or another similar toxin. Watch for the numerous signs in the station reading "Sour Gas Kills".
The toxin, which probably is seeping out of the valve site, begins to slowly build up, causing irrational behavior with the crew members. They begin to hallucinate, wander off, or attack each other.
One of the crew members (Abby) is found by a rescue team and taken to a hospital somewhere close to the drill site. When she wakes up, she finds out that the whole town is in chaos. This is an implication that the toxin seeping from the melting tundra is spreading to nearby regions, and could become a global phenomenon.
******** Common quetions: (SPOILERS again) **********
1. What are those ghost they are seeing?
In short, hallucinations caused by the toxin. They are visions of evil spirits from native Canadian folklore.
2. If the spirits are hallucinations, why is everyone seeing them?
First off, not everyone is seeing them. The expedition leader (Pollack) for example, never sees any of them. The fact that some of the crew members share the same hallucination can be explained by the fact that Dawn (who appers to be of native decent) has told them the stories of the Wendigo spirits. The more impressionable crew members begin hallucinating and their fears materialize in the form of a local legend they heard.
3. If the spirits are hallucinations, why was one caught on camera?
We have no evidence that all of the crew members saw the spirit on the camera. It could be just Hoffman's hallucination, who is already delusional and is convinced evil is coming out of the earth. Pollack behaves just as if he's seen a young man going crazy and rambling on camera. He throws the tape away and makes no mention of seeing any spirit.
Also notice immediatley after the footage is shown, we get a sow ot Hoffman staring wide eyed at the camera. This is further evidence that the spirit image from the tape is something only his eyes saw.
4. Okay, I get it, it's all just hallucinations. What's with the weird moose-dinosaur crossover?
The characters see what they want to see. They have heard the legends about the Wendigo spirits. They also talk among themselves and convince each other that they are grave diggers, and that oil is nothing but the remains of long-deceased creatures like dinosaurs. It appears that when you mix scientific climate fears with folklore and a mystery toxin, it makes for a pretty trippy ride.
5. What's up with the weather?
As the temperatures rise, the weather becomes unstable - we see rain and violent bursts of wind, one of which happens to bring the company plane down.
6. Why were people running off and taking their clothes off?
The toxin from the tundra is making the hallucinate and behave irrationally. Another explaination that adds to this is that as they wander off, they begin to suffer from hypothermia. Victims of hypothermia often believe they are hot instead of cold, and there are recorded cases of people taking their clothes off and freezing to death. This is also shown in the scene where Pollack falls in he water.