What modern phrase describes a small action that unleashes many unforeseen problems, like Pandora’s myth?

“Opening Pandora’s box” describes a small action that unleashes many unforeseen problems.

The idiom comes from the Greek myth in which Pandora opens a sealed container and releases evils into the human world. In modern English, the phrase is usually applied figuratively to a decision, investigation, technology, or argument whose consequences become difficult to control.

The expression does not normally mean that every consequence is immediately visible. It suggests that once a particular subject or process is started, complications may emerge that were hidden beforehand. This figurative use is common in journalism, politics, science, and everyday conversation.

The wording also preserves the later “box” version of the story. Hesiod’s original Greek account describes a pithos, or large jar, but the English idiom became fixed as “opening Pandora’s box.” That distinction explains why the expression and the ancient object do not match exactly.