Grand Theft Auto 5 is host to a whole bunch of easter eggs, some of which come in the form of the illuminati, who apparently worship—or at least track—a series of Unidentified Flying Objets, or UFOs, in the game.
As many of you already know, there's a UFO hovering at the top of Fort Zancudo and one that's crash-landed in to the bottom of the ocean floor.
Through the help of a map found in one of the game's locations (Mt. Chiliad), one can use it as an overlay atop the existing map of the game to discover a whole bunch of other UFOs and potentially other goodies.
Anyway, check it out below. 
Thanks Reddit.