In English idiom, what does “open Pandora’s box” mean?

In English idiom, “open Pandora’s box” means to cause many new problems.

The expression comes from the myth in which Pandora opens a vessel and releases evils or troubles into the world. In modern English, the phrase is used figuratively for an action that begins a chain of complicated, harmful, or difficult consequences.

The idiom does not usually mean that a person literally discovers a container. It is applied to decisions, investigations, technologies, or disputes whose consequences multiply after an apparently simple first step. The phrase often carries a warning that the original situation may have been easier to manage before it was disturbed.

The expression is also an example of how a myth can survive independently of its original religious setting. Many speakers use it without knowing the details of Hesiod’s account, including the fact that the original vessel was a jar rather than a box.