I have been playing F3 since it came out many years ago, and have beaten it in many differant ways. (though I dont use mods)

But the one thing I have never found is the KEY to open the cellar in the house in Springvale.

Is it actually there or is it just a tease?

AND if it IS real where do I find the key?

That cellar is the location of side-quest from Broken Steel, the quest itself can't be started until Broken Steel has (after completion of Take it Back!).

It has to do with Confessor Cromwell and the Power Of The Atom, if I remember correctly.