OK, THIS IS IN REGARDS TO THE ENDING(S). DON'T READ IF YOU HAVEN'T SEEN THE FILM...
I watched it again last night, and I am convinced Sarah got out at the end. When she falls and gets knocked out in the climax, the chamber she lands in already has some glowing daylight at the side - slow down the DVD; you can see it there already just before she lands. It gets a little brighter as she awakens and crawls out of the hole.
When she has the false start in the car and sees Juno's ghost, and seemingly awakes back in the cave, the chamber is different from the one she landed in - no daylight around, no bones.
So, I think the answer is she physically escapes the cave, but her mind is trapped down there because her psyche has devolved into something so primal - her own 'descent' - and she's crazy now. It keeps in pattern with the waking dreams and hallucinations she suffered, since the accident at the beginning.