What is the most unexplained episode?

I watched The Mountain again recently and I feel like the mountain of Mathew is probably the most deeply unexplained story line in the entire show. It’s so clear there’s something going on in that episode but the characters through which the story is told make it really unclear what’s going on. I’ve gotten SOME clarification from a bit of research but it’s still super random and is never brought up again.

Can anyone think of a more unexplained episode or story beat that isn’t just adventure time being random for the lols? (Like high strangeness or food chain I wouldn’t say are necessarily unexplained, since I don’t think there was implied to be an explanation)