What is the common English expression for opening a Pandora-like source of unexpected problems?

The common English expression is “opening Pandora’s box.”

The phrase means starting an action or investigation that produces many unexpected and difficult consequences. It comes from the myth in which Pandora opens a container and releases evils into the human world. In everyday English, the expression is usually metaphorical rather than religious: it can describe a legal dispute, a political decision, a scientific discovery, or a personal revelation.

The wording is based on the later “box” tradition, not the pithos or storage jar described in Hesiod’s Greek text. That distinction explains why the idiom can be historically familiar while still being technically different from the ancient account.

The expression is often used with the verb “open,” as in “the decision opened Pandora’s box.” It does not usually mean that every consequence is immediately disastrous; it suggests that the action releases complications that are hard to control or reverse.