I remember in season 1, there was that episode where they showed that Finn had a fear of the ocean and when ther where in a submarine and exploring the ocean, there was that ancient city like the city where we lived today and in that time humans are extinct and Finn is the only human. What if that Finn had a fear of the ocean becuase maybe the humans got extinct because a giant title wave came and wiped the whole human race. I mean it's just a theory and you don't have to agree, but something tells me that something like that happened.