Yeah, well remember I was 13 (or younger) when I did it, but the entire dungeon was originally a mine, and a section of the caves had weakened enough that an elephant had, in fact, managed to fall through the upper hill that the caves were in (HP were substracted for this even!) while walking around up there.
The problem was when I hadn't properly motivated WHY the Elephant had been walking over the hill in the first place! I'd just never thought that far ahead and the players called me on it.