Which English phrase describes an action that starts many unexpected troubles, like opening Pandora's vessel?

“Opening Pandora's box” describes an action that starts many unexpected troubles. The expression comes from the myth in which opening Pandora's container releases evils into the human world.

In modern English, the phrase is used figuratively for a decision or investigation that creates consequences difficult to control. It can apply to politics, technology, family disputes, or any situation where revealing one problem exposes many more. The expression preserves the myth's basic cause-and-effect structure.

“Opening a can of worms” is a close modern idiom, but it is not the canonical phrase derived from Pandora. “Crossing the Rubicon” means passing an irreversible point, while “meeting one's Waterloo” means suffering a decisive defeat. Those expressions may fit some contexts, but only “opening Pandora's box” directly names this mythic source.