His first real success – and probably the work for which he was to be best-known – came with The Sentinel, a short story which had appeared (as Sentinel of Eternity) in 10 Story Fantasy in 1951, and was republished in the collection Expedition to Earth (1953). It is an account of the discovery of a monolithic alien artefact on the Moon, the emanations from which come to indicate significant insights into Man's place in the universe, and into the development of the species.