Humanoid entities, probably artificial, featuring the ability to fly. These could maybe be working as explorers.

Freedom for Information Act, USA law wanted by Gerald Ford in 1974 and activated in 1976. The Freedom of Information ActFOIA) is the implementation of freedom of information legislation in the United States. It was signed into law by President Lyndon B. Johnson on July 4, 1966 (Amended 1996, 2002, 2007), and went into effect the following year. This act allows for the full or partial disclosure of previously unreleased information and documents controlled by the United States Government. The Act defines agency records subject to disclosure, outlines mandatory disclosure procedures and grants nine exemptions to the statute. Great Britain has its FOIA since 2000, while France approves one in 1978. It has to be noted most UFO files have a high degree of classification, with the result of a refusal or a very strong censorship on these same files. Also, the NSA is immune to the FOIA. 

In slang, from French "feu" (fire),  foo-fighter indicates unknown, luminous, flying bodies seen by Allies, German and Japanese pilots from 1942. Both American and British pilots have witnessed foo-fighters as well. They were considered as a German psychological weapon, yet the very same Germans called the same phenomenon "kraut-bolids", ignoring the explanation as well.

Flying Saucer Review, one of the best UFO review in the world.

From May 1946, a UFO flap extends from Scandinavia to Europe: they are cigar-shaped, with orange flames coming out from the back. Scandinavia alone receives 600 reports: most take place at night and the objects appear on radars. It seems a few landed and explosed. Some researchers link ghost rockets to foo-fighters. This particular flap is so important to attract the attention of the American Intelligence.

Greewich Mean Time.

From the beginning of the UFO abduction literature, Grays are the most popular, most reported alien species described in  cases of close encounters and abductions. Grays are the star of the pessimistic views of John Lear: evil, pear-shaped head, big black insect eyes, tiny mouth. Grays aimed at the extermination of the human race. UFO folklore says they have reduced sexual and digestive apparatus, reproducing themselves through cloning processes. More serious sources report Grays to be the working force behind someone else in search of the human soul. Whatever the truth is, the Gray extra-terrestrial has become an icon of our culture.