The modern idiom meaning that one action has released many unexpected problems is “open Pandora’s box.”
The expression comes from the Greek story in which Pandora opens a container and releases diseases, suffering, and other troubles into the human world. In modern English, the phrase usually describes an action that creates a chain of complicated or harmful consequences.
The idiom does not require a literal box or a supernatural event. It can apply to politics, technology, family disputes, scientific discoveries, or any situation in which investigating one issue exposes many more. Its meaning is figurative but remains closely tied to the myth’s image of irreversible release.
“Open a can of worms” is a similar modern expression, but it is not the same idiom. “Cross the Rubicon” refers to passing a point of no return, while “meet one’s Waterloo” means suffering a decisive defeat. Those phrases should not be treated as exact synonyms.