UFO in the sky. Computer artwork of an unidentified flying object (UFO) flying across a cloudy sky. UFOs are sometimes observed taking the form of "flying saucers". Some people believe that such objects are vehicles from outer space, carrying aliens which observe and even abduct humans. Sightings of UFOs are common, but most have a simple explanation. High cloud weather balloons, military and commercial aircraft, and even the Moon have all been mistaken for alien spacecraft. There are a few UFO sightings for which there is no conclusive explanation.