Virtual Nightmare, first published as "The Tunnel Under The World" in 1955, is a science fiction short novel that explores the theme of simulated reality and the simulation hypothesis.

The Simulation Hypothesis proposes that reality is in fact a simulation (most likely a computer simulation). Some versions rely on the development of a simulated reality, a proposed technology that would seem realistic enough to convince its inhabitants. The hypothesis has been a central plot device of many science fiction stories and films.

The novel follows the life of Guy Burckhardt who wake up from a terrible nightmares in his house in Tylerton on June 15. Guy dismisses the dream and goes to work as usual, the downtown offices of Contro Chemicals, which operates a highly automated and robot-staffed petrochemicals plant.

But something is not right; he is surrounded everywhere by loud and all-pervasive advertising jingles for everything from cigarettes to freezers. Guy Burckhardt begins to question the nature of his reality when strange events occur to him and it becomes apparent that there are many things about his world, and indeed his life, that do not hold up to close scrutiny.

The novel has been adapted for several media including "Virtual Nightmare", a made-for-TV horror film produced in 2000, and appeared in a 1956 radio broadcast episode of X Minus One. It was also produced by the BBC as a 1966 series 2 episode of the anthology series Out of the Unknown.

The novel inspired the screen script of the "Special Service" episode of The Twilight Zone and the development of the spec script of the 1998 American satirical comedy-drama film The Truman Show. It also offered inspiration for the 1999 virtual reality movie The Thirteenth Floor and the classic cyberpunk movie The Matrix.

This print edition contains other Fredrik Pohl's short stories from that era, including Simulacra (The Day Of The Boomer Dukes), Pythias, Survival Kit, The Hated and The Knights of Arthur.