I love the idea, I love the atmosphere, but the gameplay is a liiittle half-baked.
• The limited-oxygen serving as a timer is a great concept, but it drains too slowly to be a threat and is further a non-issue by how plentiful air bubbles are. Maybe if the neighboring rooms are all pitch black until you enter them could this current design be more rewarding. Currently, I can see all of the air pockets: I'm in no rush to ensure I can find air.
• Some hitboxes are inconsistent or misleading, namely the players' and the explosion tiles. The player being a 1x2 rectangle means you can go vertically through tunnels that are 1-tile-wide, but not horizontally that are 1-tile-tall. Honestly makes placing bombs kinda tricky. Further, when placing bombs down, the explosion hitboxes linger just a tad too long for my liking. Many times I found myself dying by walking into a visually-empty tile because the explosion is still technically active (the screenshake is the clue).
• I wouldn't say 600x480 pixels is really pocket-sized. That's like...normal game size in my experience. I couldn't fit that many pixels into my jeans!! D: A weak interpretation, honestly.
That all said, I do really enjoy what you've presented here. I love the fantasy-horror angle this takes, the threat being the player's own bravery and greed in finding the last scraps of gold that may or may not even be here. I would love to see this game expanded upon. Good work!