This is probably my favorite game I've played so far. Perfectly balance, as all things should be. Really good use of the theme, counting down is truly the core mechanic. I played for so long that I forgot rescuing your friend was the win condition lol. Reminds me of the game Motherlode I used to play on miniclip as a kid. If I had one complaint it would be that once you get to the lower strata it takes quite a long time to get back to the surface. This is somewhat alleviated by the fact that once you have enough upgrades, drilling strait up costs pretty much nothing but it still takes a while. A possible solution would be if the player full clears a strata, they only have to go back to the strata above that one. That way the player is incentivized to full clear, though not having to spend as much time going to the surface might mess with the balance. The game is so well balanced already though that I have faith you could come up with something that works haha.