In Haunt #22, The Abyss Gazes Back, the house is collapsing into The Abyss.
The traitor is sometimes allowed to choose the next room which will collapse into the abyss. One of the ways they can do this is with the following rule from the Traitor's Tome:
If the Abyss engulfs a whole floor, it moves to the next floor up, starting in an unoccupied room of your choosing with an unexplored door.
If there is no such room available, which room should be collapsed by the traitor?