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.
There should be a burrowing hole in a passage on the left (as you come from the bridge). Dig underground and smack the switch to open the gate, then go back topside and go through the passage. You'll be in a room with some Water Fruit. Hit the switch to open the door and take a Water Fruit with you, then turn right and jump down. Kill the Magmanos and go through the passage unlocked and there's the chest.
You can watch a video of this here.