On July 25, 1976, NASA's Viking 1 Orbiter spacecraft was searching for a landing site for the Viking 2 Lander. As part of this search it photographed the Cydonia region in the northern latitudes of Mars.
One image of this region of eroded rock, buttes and mesas shows a peculiar feature about one mile (1. 5km) across, which looks very clearly like a human face.
More properly it resembles a simple facial mask, with one eye, a nose, a mouth with slightly parted lips and a chin.
The right half of the 'face' is in shadow, which prevents us from judging symmetry, but the effect is striking and uncanny. The overall picture is riddled with small circular drop-outs caused by transmission errors, one of which puts a 'nostril' in the proper spot along the ridgeline that forms the 'nose'.
NASA explained the image as merely an artifact of light, shadow and the human desire to pattern match (Pareidolia).
Nonetheless it was quickly seized upon by sensationalists, tabloids, conspiracy theorists and others as an "alien artifact." All this was great fun, and NASA decided in 2001 that another picture of the region would be in order.
NASA rolled the Mars Global Surveyor spacecraft into position on April 8, 2001 to take a few shots of the region.
These shots, taken from another angle, showed the 'face' as seen from 165 km to one side at a total distance of about 450 km. Seen from the new angle, the features that make up the face are clearly geological, and no artifact exists.
Or so NASA says. Actually the new picture looks a heck of a lot like Neil Armstrong's bootprint on the surface of the moon... and both US and Russian Mars space probes keep disappearing, including the Mars Observer, lost in 1993 on orbital insertion, and most recently (1999) the Mars Climate Orbiter, just as it, too, entered Mars orbit.
Clearly NASA is not so careless. Could the Face be shooting them down?
See http://www.ntskeptics.org/1998/1998april/april1998.htm for the original and new images.