What is the common English idiom for an action that unleashes many unexpected problems?

“Opening Pandora’s box” is the common English idiom for an action that unleashes many unexpected problems.

The expression comes from the Greek myth associated with Pandora. In Hesiod’s story, Pandora opens a vessel and releases illnesses, suffering, and other troubles into the human world. Modern English turned that narrative into a metaphor for beginning a process whose harmful consequences are difficult or impossible to control.

The idiom is normally used figuratively in journalism, politics, law, science, and everyday conversation. Someone may say that a decision “opened Pandora’s box” when it created a chain of complications beyond the original intention. The phrase does not require a literal box or any connection to Greek religious practice.

The wording reflects later retellings rather than Hesiod’s exact description. The earliest Greek accounts describe a pithos, a large storage container. Renaissance and later European writers helped shift the image toward a box, and that familiar form became fixed in English usage. The related phrase “Pandora’s box of problems” is a variation, not a separate myth.