"Time and Time Again" is a science fiction short story by American writer H Beam Piper, first published in April 1947, Astounding Science Fiction magazine. 

"The Tunnel under the World" is a science fiction short story by American writer Frederik Pohl, first published in 1955 in Galaxy magazine. It has frequently been anthologized, for example in The Penguin Science Fiction Omnibus (1973) edited by Brian Aldiss, and The Golden Age of Science Fiction anthology edited by Kingsley Amis (1981).