英语描述UFO

We all know that we live in a complex world .Although the technics of the human beings are developing all the time , we still got so many unsolved problems. Among all of this puzzles , UFO is one of the most noticable things.
Now as everybody knows ,UFO is what we call Unknown Flying Object for short.For a period of time ,once we mentioned UFO , we always communicate it with high-tech ,extra-terrestrials ,outter space culture and many other exciting things .
There are a lot of witnesses around the world ,affirming that they chanced to have seen some strange things in the sky . However , most of them cannot hand in the credible evidence .That's a pity.
With the long-term efforts of the ufologists , most of the incidents are finally considered as natural phenomenons or human conducts . Some so-called UFO is just a bird , a plane , a balloon or even a plastic bag .
Until now ,we haven't found any clues on real extraterrestrial civilizatio