UFOs and Aliens
The idea that Unidentified Flying Objects in the skies of the world might be spacecraft from another place has been a going topic publically since a strange sighting that was reported in 1947.
Since that time, beliefs and theories regarding the UFOs has come to include claims of historic and ancient sightings, encounters with alien beings, and cover-ups perpetrated by a number of individuals and organizations... in short, it's become a complicated mix of evidence, beliefs, and theories, with very little actually sorting the mix out. Which is why I don't often examine claims in this topic, by the way.
Still, individual cases stand out as defining some of the beliefs and theories that have developed, and some cases have straightforward evidence, no matter how anyone chooses to interpret it afterwards. So I do make exceptions for unusual cases with both a greater influence and better information.
Oh, and one last thing: I use the term "Alien" here in the classic sense of 'foreign to the environment'... I do not intend to imply any specific point of origin for a being when I call it an Alien, other than the collective shrug of "not from here."