I have noticed a cellar door in one of the remains of a house in springvale but can't for the life of me find this key to open the door. I also experienced this door not being there at another stage of the game, I think it was in the earlier parts and now it is there. Does anyone have ideas where this key could be to this mysterious cellar door? thanks.

It's added for a quest in Broken Steel. You'll be able to access it in a sidequest once you've hit the Broken Steel Content- ask the nice guy at Project Purity about Megaton's increased demands for water.

nothing down there but ghouls and i think some non hostile ferals but i went down there with a mask on so :shrug:

They are non-hostile, but if you go down there when you're supposed to, you'll fine Mother Curie III as well. You'll need to talk to her or kill her to complete the quest; attacking her turns them all hostile.

Ok, that explains whats going on here. thanks everyone.