Area 51

Area 51 is a military base in Nevada, and a former testing ground for experimental government aircraft and extraterrestrial phenomena. The area 51 appears to contain some alien technology, like a captured flying saucer re-captured by aliens, but crashed in California.

Appearances
Area 51 is mentioned in Fallout, as a print on a flying saucer in the alien ship special encounter. It was to appear as a location in Fallout 2 and be inhabited by a group of scientists, but was cut for the final version of the game.

Behind the scenes
In one of the first versions of Fallout 2, Lynette's talking head was supposed to be used for a spokesperson for a group of scientists living in the ruins of Area 51.

Despite the fact that the Mojave Wasteland is featured as the main setting of Fallout: New Vegas, Area 51 is never seen or even mentioned anywhere in the game, even with the Wild Wasteland trait active.