We're used to seeing flying saucers and creatures from outer space on TV, but have you ever seen them in real life?
Well, some people think they have, and for the first time the government's released secret files of unidentified flying objects (UFOs).
The files contain letters and drawings by people like pilots and policeman.
But experts don't think the strange sightings were actually aliens, but were probably caused by lights from things like aeroplanes and satellites.
Even though the files don't prove there's life on other planets, they're really interesting because they show what sort of weird things people think they've seen.
One man says he saw a triangular shaped craft moving really fast through the sky, when nothing was reported to be flying in the area.
And some UFO spotters are convinced they've seen alien life.
They say stories haven't been investigated properly, and are convinced the government's trying to cover up the truth.