So, in short, my suggestions are:
1. More input. Much of the vital info about the game is not in the game itself, but in this page. Maybe a couple more dialogue lines would help let the player know they're on the right track. Maybe make it more explicit that we're looking for a sign from the Geiger counter, or trying to reach the very bottom in particular.
2. An impulse command might make navigation less frustrating, especially after having started over a few times.
3. A way to replenish some oxygen. That might also give us more to interact with using the E key, adding consistency to the gameplay. Currently, it seems to me that there is not enough room, or time, for exploring and making mistakes.
I have a few more ideas other than that, mostly to add to the atmosphere.
Overall, I like concept and execution of this game and would like to see more. Sound effects are very well executed and so is the ambience, both in model, texture and effects, especially light range. It definitely has potential.