Alien UFO Easter Egg Location: Area 51
To find Area 51, travel down to the Mountain States region. The base is right on the edge of the Mojave Desert, right as you enter the Western Shrub-Steppes.
One way to find the base is to start at the "Western Shrub-Steppes" title on the map and travel southeast. The big silver UFO is walled-off inside the army base.
There doesn't seem to be any easy way inside, but the UFO is clearly visible through the fences. Go into first-person view and look through the uncovered exterior to get a good look at this alien oddity.