The VG Resource

Ooh, sounds like the nether in Minecraft. And I'm glad the vertical doors act like air ducts when you go through them; solves a pretty big issue that the Zelda 2 dungeons brought up, that being there's too much horizontal traversal. I probably would have used ladders, but ducts add variety.

[EDIT]
Noticing how frequently it happens, I swear, I'm not being the first on new pages on purpose. XD

I really am enjoying the progress in the game. Also the tileset of the underground area looks very similar to the super mario world cave area.