Demons

This is one of my favourite theories for the origin of UFOs. Yes really.

Aliens are but one explanation of which others include "intraterrestrials" who live in a hollow earth accessed by a hole at the North Pole. "Ultraterrestrials" are beings from another dimension though I am not really sure how that works. And there is the theory that it is all a big cover up to distract us from what is really going on (whatever that is). Personally I quite like the idea that the denizens of hell are fighting their unearthly battles above our heads as we go about our business, perhaps leaving behind a whiff of brimstone as they go. Better than boring old aliens anyway.

There is also the Null hypothesis which suggests that it is all a load of old marsh gas.