The Sentinel
Arthur C. Clarke
 The story deals with the discovery of an artifact on Earth's Moon left behind eons ago by ancient aliens. The object is made of a polished mineral and tetrahedral in shape, and is surrounded by a spherical forcefield. The narrator speculates at one point that the mysterious aliens who left this structure on the Moon may have used mechanisms belonging "to a technology that lies beyond our horizons, perhaps to the technology of para-physical forces." The narrator speculates that for millions of years (evidenced by dust buildup around its forcefield) the artifact has been transmitting signals into deep space, but it ceases to transmit when, some time later, it is destroyed "with the savage might of atomic power". The narrator hypothesises that this "sentinel" was left on the moon as a "warning beacon" for possible intelligent and spacefaring species that might develop on Earth. In 2001: A Space Odyssey, the operation of the sentinel is reversed. It is the energy of the sun, falling for the first time on the uncovered artifact, that triggers the signal that creatures from the Earth had taken the first step into space.