In this story by Arthur C. Clarke, a scientist exploring the moon’s surface discovers a “glittering roughly pyramidal structure” high on a mountain top. It is clearly not man-made, and is surrounded by a force-field that takes twenty years and “atomic power” to penetrate. The scientist concludes that it was placed there as a beacon to alert an advanced alien civilization when life on Earth has evolved and proved worthy enough for them to be interested in revisiting. This begs a question: what are their intentions? Themes include isolation, exploration, discovery, evolution, worthiness, the cosmic insignificance of man.
