I like the Jordan curve theorem, because apparently no one even thought of it as something that needed to be proven until the 1880s, and then it turned out to be remarkably difficult to prove.
The statement of the theorem can basically be summed up as "every closed, non-self-intersecting continuous curve in the plane has an inside and an outside." which is glaringly obvious
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@Austin_Dern Yeah, that pretty much sums it up.
I also am reminded of a quote, I don't remember from where, "The axiom of choice is obviously true, the Banach-Tarski paradox is obviously false, and who can say about Zorn's lemma?"
The punchline being that all three of these things are equivalent; each one implies the others.
