What is the common English idiom for an action that creates many unexpected problems, based on Pandora’s myth?

The common English idiom is “opening Pandora’s box.” It describes starting an action that releases numerous unforeseen problems or complications.

The expression comes from the myth in which Pandora opens a sealed container and allows troubles to escape into the world. In modern usage, the phrase is usually metaphorical rather than a reference to Greek religion. It can describe a political decision, a technological development, or a personal action whose consequences become difficult to control.

The wording is itself historically layered. Hesiod’s original story describes a pithos, or large jar, while the “box” formulation developed through later translation. Despite that change, the idiom has become firmly established in English.

“Opening the floodgates” is a related metaphor, but it is not the specific expression derived from Pandora. “Crossing the Rubicon” instead refers to an irreversible decision, while “meeting one’s Waterloo” means suffering a decisive defeat.