This is as far down as you can go.
The GRAND FOYER is still a few floors down, but the TRANSPORTALIZER on that level is blocked by one of GRANDPA'S impressive BIG GAME TROPHIES, and you just don't think he would cotton to someone moving it.
Speaking of which, here are some of his TROPHIES now. He has a million of these ghastly things. You really dislike them.