“Open Pandora's box” means to start a process that releases many unforeseen problems.
The expression comes from the myth in which opening a sealed container allows suffering and troubles to escape into the human world. In modern English, the phrase is usually figurative. It can describe an action, investigation, policy, or technological change whose consequences become difficult to control.
The idiom is related to, but distinct from, “the genie is out of the bottle.” Both expressions suggest that released consequences cannot easily be contained, but Pandora's box specifically invokes the Greek myth and its collection of evils. The wording also preserves the later box version rather than Hesiod's original jar. Its broad use in journalism, politics, and everyday speech has made the mythological reference familiar even to people who have never read Hesiod.