So Long, an Thanks for All the Fish (1984, ISBN 0-345-39183-7) is the fourth book of the Hitchhiker's Guide to the Galaxy series written by Douglas Adams. Its title is the message left by the dolphins when they departed Planet Earth just before it was demolished to make way for a hyperspatial express route, as described in The Hitchhiker's Guide to the Galaxy. The phrase has since been adopted by some science fiction fans as a humorous way to say "goodbye" and a song of the same name was featured in the 2005 film adaptation of The Hitchhiker's Guide to the Galaxy.