What modern idiom describes releasing many unexpected troubles by opening Pandora’s box?

The modern idiom is “opening Pandora’s box.”

The expression describes taking an action that creates many unexpected problems or releases consequences that are difficult to control. It comes from the Greek myth in which Pandora opens a storage vessel and troubles escape into the human world. In everyday English, the phrase usually applies metaphorically to a decision, investigation, or technology that produces a cascade of complications.

The expression is based on a later translation tradition. Hesiod’s original Greek word refers to a large jar or storage vessel, not a small modern box. The “box” version became dominant through influential retellings and translations, especially in European languages.

“Opening a can of worms” has a similar meaning, but it is a separate idiom. “Crossing the Rubicon” means passing a point of no return, so it is related to irreversible action but does not specifically refer to a flood of troubles.