Oh man, i'm surprised at the hate for The Descent. I think this movie is tremendous, pretty much from top to bottom. And, while admittedly the whole "horror movie or not" theme springing up in the comments, and on the forums, (which I've totally participated in) is getting pretty tired at this point, i'm totally flabbergasted that anyone would argue this isn't a proper horror movie.
From the horrifying suddenness of the car accident, to the bizarre scene of Sarah on the floor of the hospital hallway, people walking right by her like she's not there, to the jump scare hallucination in the cabin, to the soul-crushing weight of the cave itself, to the whispering of her dead child, man, this sucker is almost a clinic on how to craft a horror movie. And that's all before the monsters show up.
And when they do? Fucking hell, things go sideways pretty quickly for the ladies in the cave.
There is so much craft that goes into developing the characters and their relationships, hinting at subtle fissures between Juno and Sarah, suggesting with looks and comments what people know, and what they don't, about the affair, and Sarah's mental health. While it's not on the scale of The Thing, i'm reminded of how Carpenter spends some time and gives space for his characters and their relationships to develop before throwing the monster into the mix.
Also, the thing about Juno, the woman she axed (Beth?) was not dead, she was pleading with Juno not to leave her there, and Juno being Juno, left her behind, fled in a time of need. Which is exactly what she did to Sarah after the accident. She's confronted by another one of the companions earlier about how she wasn't there for Sarah, how she ran away.
And Sarah, well Sarah is obviously still emotionally scarred, and when she eventually rises out of the blood, I read that as her having completed her descent (I know, that's cheesy), and lost the remaining shreds of her humanity. She becomes more like the creatures in the cave. That could be a weak interpretation, but I'm not sure how else to explain her treatment of Juno at that point. It was a pretty shitty thing to do, that axe hit to the leg.