The Sentinel is a
short story by
Arthur C. Clarke which tells of the discovery of a object on the
dark side of the moon, placed there by an
alien civilisation, and protected by an
invisible force field. It's purpose is to
test whether
Man has achieved two things: mastery of the
atom (the force field is disrupted by a
nuclear device) and mastery of
space flight.
The Sentinel is opened, and it begins to transmit that humans have become technological.
An otherwise unremarkable story, except for the fact that it spawned 2001: a space odyssey.