Graphics are really eye-catching, and the impending doom of the shrinking visibility and audio that goes with it really works to get on one's nerves.
I was a little torn on the reduction of visibility, since it eventually gets to a point where the player can't really win anymore, since they can't see anything (unless they happen to already be at the right place). However, when I did finally make it to the end (try 3, I think?), the panic as the light got snuffed out but the continual upward movement really worked well to evoke the feeling I imagine you wanted players to feel. And that feeling when you see the end -- great stuff.
One thing I'd suggest is giving some kind of indication as to whether the player is still moving. When it gets real dark, you might just be swimming up into a ceiling, and there's no way to know.
Also, there were a few places where it was difficult to distinguish where you can and can't go; as in, it looks like there's a wall, but it's actually just a background asset. Having the background objects be darkened or lightened or something to distinguish them would be helpful. On that same note, there are a few places where it looks like there's a gap between two rocks, but then my guy can't manage to pass through; having that look impassable would have been appreciated.