What is the standard English idiom for an action that releases many unforeseen problems, based on Pandora’s myth?

The standard English idiom is “opening Pandora’s box.”

The phrase describes an action that creates or reveals many unforeseen problems. It comes from the Greek myth in which Pandora opens a container and releases evils into the world. In modern English, the expression is usually applied figuratively to a decision, investigation, technology, or policy whose consequences become difficult to control.

The idiom developed from later retellings and translations of the story. Hesiod’s original vessel was a pithos, a large jar, but Renaissance translation helped establish the word “box” in European languages. The phrase became especially familiar in English through literary and journalistic use.

“Opening a can of worms” is a related modern expression, but it is not the same phrase. “Crossing the Rubicon” refers to an irreversible decision, while “opening Pandora’s box” emphasizes the release of multiple unexpected troubles or complications.