Taking this seriously though:
1) When I was a kid, me and 3 other kids who were my friends seriously thought we were vampire hunters. There were aliens and vampires in the woods behind our house and we all saw them. Did we really though? Of course not.
2) Let's take it more seriously though. What if they actually did see something? The guy that jokingly said "swamp gas" might be right. Remember the Oracle of Delphi from the movie 300? For hundreds of years, Greeks would go there, and they would experience the same visions of the same prophecies.. Were they really experiencing prophecies together? Or is it just a coincidence that there's a natural gas leak there and the entire area is covered in gas that causes hallucinations? Hmmm.. So, if they really saw something -- what's more likely: 1) aliens traveled across the galaxy to show psychic videos of global warming to kids on the playground, or 2) some kind of similar mass hallucination, maybe actually "African jungle gas" instead of "swamp gas"? What was true when the same thing happened a few thousand years ago in Greece, 1 or 2? There's your answer.