"The Sentinel" is a science fiction short story written by Arthur C. Clarke. It was first published in 1948 and explores themes of extraterrestrial life and human evolution. The narrative centers around a mysterious monolith discovered on the Moon, which serves as a signal to advanced alien civilizations. The story suggests that the monolith was placed there by an advanced race to monitor humanity's progress. As the protagonist reflects on the implications of this discovery, it raises questions about the nature of intelligence and the future of human exploration in the universe.