I've seen 3 of them in my life, one scary/odd one I saw above my neighborhood as a child "hovering"(not moving like anything else I've ever seen, it looked like it just wasn't subject to the same physics everything else has to abide by) in the sky - I specifically didn't tell anyone about it because I was scared and I thought the adults' reaction might scare me even more (like if someone made "x files" noises or if they said it was a UFO, that would have made the situation worse), so I didn't say anything and didn't think of it again until years later - but the experience has always stayed with me and I can still recall it clearly almost 20 years later. The thing looked for all the world "like an alien space ship", not like the ones you'd seen in scifi tv series or movies, but like it was a legitimately "alien" looking "ship" or object. The "ship" had aspects that made it look like it was a living organism itself (but no, it wasn't a bird or anything like that, not even close - it was like nothing I had ever seen before or since). It was the like the material used to build it or coat it absorbed the light rather than reflecting it. The way it moved and just stayed there for what seemed like hours(but was really probably more like 20 minutes) with no one noticing it or making a fuss, right above a peaceful suburban neighborhood, still gives me goosebumps to this day.
The second and third I saw as an adult in the sky above my house, clearly and within moments of each other. After this I decided to do some reading, and found out that:
US and Russian astronauts and pilots say they're aliens and some say they've had direct professional experience with the subject while in the service, one (Gordon Cooper) has testified about it to the UN and has briefed officials in DC about it, (6th man to walk on the moon, Dr. Edgar Mitchell.
Russian cosmonaut / Soviet Air Force Colonel Marina Popovich says the KGB and Soviet Air Force has recovered these UFOs on multiple separate occasions over the past century -
Marina Popovich, a retired Soviet Air Force colonel, engineer, and legendary Soviet test pilot who holds 102 aviation world records set on over 40 types of aircraft. She is one of the most famous pilots in Russian history, and one of the most important female pilots of all time.
Marina Vasilieva became a Soviet Air Force pilot and in 1964, a military test pilot. She authored nine books and two screenplays. Among many other awards, she has been honored as Hero of Socialist Labor, the Order of Courage (presented personally by Vladimir Putin in June 2007)
She claims that the Soviet military and civilian pilots have confirmed 3000 UFO sightings and that the Soviet Air Force and KGB have fragments of five crashed UFOs. The crash sites were Tunguska (1908), Novosibirsk, Tallinn, Ordzhonikidze and Dalnegorsk (1986).
She speaks about her experience with UFOs in her book titled UFO Glasnost (published in 2003 in Germany) and in public lectures and interviews. These 3 are a tiny sampling of the highly trained, credible people in relevant responsible professions (defense/aerospace/military/aviation/science/etc.) from all around the world who have come out trying to 'blow the whistle' and inform humanity of something they think of as important and worth knowing about. There are others like FAA Head of the division of Accidents & Investigations, John Callahan says the same thing, claims he was involved in the 'cover up' and was told by guys from the CIA to keep quiet, 'this meeting never happened' etc. after taking all of his evidence away, luckily he had made copies (of things like radar tapes, etc.) and he's now trying to inform people, but you've probably never heard of him.
Some others to look into would be Dr. Milton Torres (former USAF fighter pilot, now university engineering professor, who says that while in the air force he was ordered to intercept a UFO the size of an aircraft carrier that was orbiting above Ipswich, ordered to fire all his missiles at it, and when he saw it and had radar confirmation/target lock, the UFO shot off at ridiculous speeds and was immediately off his radar, only remaining on the longer range ground radar for another 3 sweeps before going out of range - so the speed can be estimated at "what the fucking shit" MPH), Colonel Charles Halt and the Bentwaters incident (as well as all the other USAF personnel who were there), the Belgian Air Force general who went on TV talking about their UFO incidents, just google "military UFO whistleblowers".