The face on Mars
The routine web-surfer in me was indulging in some idle timepass, when I came across the wikipedia article regarding the face on Mars, which is actually not a face at all.
Just some low resolution photography, combined with the opportune lighting attributed to when the photograph was taken and most vitally, the phenomenon of "Pareidolia" contributed to everyone freaking about alien presence on Mars.
If you've seen this really decent movie titled "Mission to Mars", starring Gary Sinese, the face plays an important role by itself, in the movie.
http://www.imdb.com/title/tt0183523/
In addition, for those readers who actually have come this far, Pareidolia is supposed to be the human tendency to mistake and correlate random shapes as discernible shapes. Pareidolia is that phenomenon that probably took you on wonderful flights of fancy on a mildly cloudy day when you sat along with that someone special and were able to see heart shaped clouds in the sky, or maybe dragons snorting fire on a medieval castle, burning it half down (if you were lucky enough!) .
Check out the wikipedia article on the same: interesting, it is!
http://en.wikipedia.org/wiki/Pareidolia
Finally, Futurama (episode ???) portrays the face of mars as the entrance to the center of the planet where the ancient Martians live. It is accompanied by the "Great Ass of Mars", located on the other side of the planet.