The Descent is unquestionably one of the greatest horror movies made since 2000, which still holds the power to shock and terrify over a decade after its release. Still, depending on where you saw that release, you may have missed the truly great ending.
The US cut ends at 3:22, with Sarah spooked by the bloodied ghost of Juno next to her in the car. However, the UK ending has an extra portion added on. If you've never seen it, check it out!
In the US theatrical release of The Descent, Sarah wakes up in her car and is scared by Juno's ghost. We see one shot of Sarah's eyes, wide with terror, before the credits roll. In the UK release (and on the uncut DVD version) we get a more ambiguous version.
It turns out that Sarah is still underground, crawling in a creepy, disconnected way that Kayako would be jealous of, suggesting that she has started to take on the mannerisms of the creatures she's been running from. She sees her dead daughter, Jessica before her, holding the birthday cake she would have had had she not been killed.
The camera pulls away and reveals that the flame she sees is actually no more than her torch on the ground, but Sarah gazes ahead, in the thrall of her hallucination. She never got out, her mind just broke.
I personally like the dark tone of the extended ending, and think it fits better with the pessimistic tone of the rest of the film. Perhaps test audiences demanded a more positive ending? Brian Collins has a nice way of looking at the extended ending:
It is, in purely scientific terms, a giant fucking bummer.
Or is it? If you think about it... the US ending was inadvertently made darker by removing this epilogue and presenting her escape as reality. If she escapes, she's completely traumatized, now all of her friends are dead (she even contributed to two) in addition to her family, and she's gonna have to answer a lot of questions about the disappearance of five people. At least if she's still trapped in the cave, she's mentally at peace, with her fractured mind believing that her daughter is with her. It's the first time I watched the movie since having a child of my own, and thus the first time I really appreciated how much "happier" this ending is - even if I had proof of my innocence and a few surviving friends, I'd still choose to exist in a world where my son was still with me rather than return to the reality where he wasn't.