This was a great game, only thing that disappointed me was the short length. The controls were a bit awkward, however I got used to them eventually. Only thing that bugged me were the ball causing the elevators to get stuck and move slowly if you're touching an edge.
I loved the atmosphere. The art was nice and the writing on the walls was a great touch. Discovering all the messages made exploring more enjoyable and they were a pretty powerful way of telling the story. The spooky audio worked well too.
I liked how we got a glimpse of the surface and then it made us drop deep into the ground. It was both cruel and brilliant. A great way to break up the pacing and add some interest to the story and visuals.
The darkness worked well. It made simple puzzles seem much harder than they were.
I think you've got a good mechanics, good pacing and a good environment/story in this, if you make it longer this is going to be awesome!