Editing The Sentinel (short story) (section)

== Plot ==
The story is set in 1996 (at the time of writing, several decades in the future). The narrator, Wilson, is a veteran [[Geology of the Moon|selenologist]] on an expedition to explore the [[Moon|lunar]] surface. He comes across a mysterious pyramidal structure, surrounded by an "[[Force field (technology)|invisible shield]]" which blocks all attempts to approach. The structure is found to have been placed on the Moon several billion years earlier, and it is not the product of any terrestrial or lunar civilization.

Twenty years pass before the invisible shield is successfully breached, using [[atomic energy]]. The structure is destroyed in the process, and its fragments are found to be incomprehensible, being far more advanced than any human technology. Wilson realizes that the structure was created and placed on the Moon by technologically advanced [[extraterrestrial life|aliens]] from outside the [[Solar System]].

Wilson presents his guess as to the structure's purpose: it was a "sentinel" which for billions of years had been transmitting signals into space. The sentinel's destruction—which humans could only accomplish by mastering space travel and atomic energy—will have caused its signals to cease, thereby alerting its creators to intelligent life having discovered it. Wilson expresses his belief that the aliens will soon be "turning their gaze upon the Earth".