“Open Pandora’s box” is the modern English phrase for starting a chain of unexpected troubles.
The expression comes from the myth in which Pandora opens a sealed container and releases harmful conditions into the human world. In contemporary usage, it usually warns that investigating, changing, or revealing something may create many complications that are difficult to control.
The phrase is often used in journalism, politics, science, law, and everyday conversation. It does not necessarily claim that the original action was malicious; it emphasizes that the consequences may multiply beyond the person’s intentions or expectations.
A related expression is “open a can of worms,” but that phrase is not identical. “Open Pandora’s box” specifically invokes the Greek myth and suggests a broad set of unforeseen problems. “Cross the Rubicon,” by contrast, refers to passing a point of no return, not releasing numerous troubles.