From _SCIENCE_, Vol. 261, Sept. 10, 1993, p. 1392:

A Face-Off on Mars

    Although the silence from the Mars Observer was deafening, and the
atmosphere inside the Jet Propulsion Laboratory (JPL) was one of quiet gloom
during the crisis over its missing spacecraft, outside things were a bit
noisier.  Two days after JPL lost contact with Observer, picketers were
marching in front of the building, protesting that NASA had purposely blacked
out the craft.  Why would the beleaguered space agency do such a thing?  To
cover up evidence of exterrestrial life, of course.  "NASA Face Up To It" read
one sign, as the picketers claimed that the agency was afraid to take new
images of the famous martian "face" that the Viking mission had photographed
back in 1976.
    Pictures beamed back from Viking that year showed a huge, mile-wide rock
formation on the Red Planet that looks somewhat like a somber, sphinx-like
face.  To some observers, computer-enhanced analysis of Viking's pictures
suggested the face wasn't natural, but deliberately constructed, and thus
another obvious remnant of an ancient civilization.
    Psychologists agree that the face is an artiface--but one created by the
eyes of the beholders, not by martians.  They say it's a textbook illustration
of the natural human tendency to see faces given minimal detail.  "The human
visual system is organized to see [features] in random patterns, and faces are
particularly salient," says University of Rochester psychologist David
Williams.
    To this point, NASA has not bothered to respond to the recent protest.
They're still too busy searching for the Mars Observer to look for a mysterious
counenance.

