This was pretty fun to play but one thing really dragged me out of that. I don't know if it's a problem on my end or in the code, but the frame rate absolutely plummeted when I would surface after a deep dive. I had to dive back down and resurface two or three times before it would smooth out.
Also, I wish that the player sprite could fit down a single square, because I wanted to mine straight down to see how far I could get, or make a really quick surfacing chute. I wanted to slingshot to the moon!
That all being said, I did really like how the surfacing mechanic worked, that was pretty cool. I was honestly more interested in doing goofy things with that than the mining and upgrading (jumping over the upgrade shop and whatnot). It may honestly be a different game, but giving the player things to mine up in the sky too could be fun, building up momentum then bursting up there.
I enjoyed the music, it gave it a very nice vibe and wasn't distracting. I think you did a really good job balancing the audio too, because when the O2 alarm went off I didn't jump out of my seat, I calmly proceeded to the surface.
Impressive work!