Pandora's box

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 given to Pandora which contained all the evils of the world.

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.