Pandora's box is an artifact in Greek mythology, taken from the myth of Pandora's creation in Hesiod's Works and Days. The "box" was actually a large jar (πίθος pithos) given to Pandora (Πανδώρα, "all-gifted", "all-giving"), which contained all the evils of the world. Pandora opened the jar and all the evils flew out, leaving only Hope inside once she had closed it again. Today the phrase "to open Pandora's box" means to perform an action that may seem small or innocent, but that turns out to have severely detrimental and far-reaching consequences.
See Special:Whatlinkshere/Etymology:Pandora's box for a list of articles using this term.