text:  They might be the remains of a very ancient human civilization that survived some catastrophic event and may remain hidden from us. And finally, they suggest some sort of supernatural beings, maybe fairies or elves, that are hidden from us, which, given our warlike nature, can't be seen as much of a surprise. For me, the best solution for the UFOs is they are off-world craft flown by non-human pilots, but then that's just me.