What modern expression means starting a chain of difficult consequences that cannot easily be reversed?

“Opening Pandora’s box” means starting a chain of difficult consequences that cannot easily be reversed.

The expression comes from the myth in which Pandora opens a container and releases troubles into the world. In modern English, it is usually used figuratively rather than literally. A person may “open Pandora’s box” by asking a question, revealing a secret, or taking an action that creates many unforeseen problems.

The phrase is often confused with “opening the floodgates,” which suggests allowing a large quantity of something to flow freely. The Pandora expression specifically emphasizes unexpected harmful consequences and the difficulty of restoring the previous situation.

Although the ancient story concerns a jar, the English idiom uses “box” because later translation helped establish that form. The idiom is now much more common than the literal mythological wording in everyday speech.