My personal belief is that UFOs are native to this world. There have been many races of humans on the planet over the past couple million years. We're the last ones left, but what if we're not? I'm not an advocate of the "Hollow Earth Theory" or anything, but there's a shitload of ocean out there, and a lot of UFO sightings have been seen over the ocean.
If they're NOT another race of humans, perhaps they're actually life forms that are natural to this world. I don't know if any of you have read "The Mothman Prophecies" by John Keel but he delves into the subject of UFOs and the paranormal and comes up with a pretty believable explanation.