The Phobos "monolith"
The account
A rectangular structure on Phobos was thought to be an artifact left by someone, and even an Apollo astronaut talked about it.
What investigators found
It is a large, isolated boulder, probably thrown there by an impact, casting a long shadow in the photos.
The story
In 1998, the Mars Global Surveyor probe photographed Phobos, the larger moon of Mars, and one of the images showed a bright spot with a long shadow, looking like a stone standing upright. The object earned the nickname monolith, a nod to the film 2001: A Space Odyssey.
Its fame grew when Buzz Aldrin of Apollo 11 mentioned the monolith in a TV interview and said Phobos would be worth visiting. Edited clips of his remarks still circulate as if he had revealed a secret, but the astronaut was talking about a geological curiosity. To scientists, it is a large block of rock, perhaps thrown there by the impact that formed a nearby crater. The elongated shadow, cast by the low Sun, exaggerates the impression of a rectangular shape.