Some ufos may be due to natural phenomena which we have yet to fully understand.
I recall the "foo fighters"(
Foo fighter - Wikipedia, the free encyclopedia ) have been linked to earthquake lights(
Earthquake light - Wikipedia, the free encyclopedia)(
YouTube - 30 mins before the 2008 Sichuan earthquake in China )(there was apparently seismic activity in the area about the time they were seen).
Also, I have seen what is usually described as "ball lightening" in my house -a white glowing ball about the size of a grapefruit, about two foot off the floor moving through a room and disappearing through a wall - this was witnessed on two occasions by a visitor, who was as goggle-eyed as myself. This occurred for three nights in succession and then stopped (I should say my house is NOT built on the site of an old Native American burial ground!).
Obviously not a ufo,but an un-expained occurence, and one which I would dismiss as imagination if I had not seen it myself.