I completed the Fire Sanctuary in Zelda: Skyward Sword and after going back inside I found I never visited the Basement Floor of the Dungeon Map. There is a treasure chest I had not gotten. I searched all over for an entrance to get there and could not figure it out. Has anybody else noticed this?
There is a specific chest in the Fire Sanctuary that requires a decent amount of backtracking, so I will assume this is what you're looking for.
You will need to go to the first outdoor bridge you crossed, where you fought a Dark Lizalfos. Go through the door on the east side (the one leading towards the dungeon entrance) and look for a spot to dig into nearby.
You can watch a video of this here.