What modern English expression means to unleash many unexpected problems?

“Open Pandora’s box” means to unleash many unexpected problems.

The expression comes from the Greek myth in which Pandora opens a container and releases the troubles held inside. In present-day English, the phrase is usually used metaphorically rather than literally. It describes an action that may appear limited at first but produces a cascade of difficult or harmful consequences.

The idiom is related to, but not identical with, the ancient myth. Hesiod described a pithos, or large jar, and his account is concerned with the origins of human suffering. Later translation and retelling changed the jar into a box, creating the familiar wording. The modern expression preserves the central action of opening a sealed container.

Writers use the phrase in discussions of politics, technology, science, law, and everyday decisions. It does not necessarily mean that every result will be disastrous; it emphasizes that opening the issue may release problems that are difficult to control or reverse.