What modern expression means introducing a problem whose consequences cannot easily be reversed?

“Opening Pandora’s box” means introducing a problem whose consequences cannot easily be reversed.

The expression comes from the Greek myth in which Pandora opens a sealed container and releases evils into the world. In modern English, it is used figuratively when an action reveals a complicated set of troubles or triggers consequences that are difficult to control.

The idiom is not limited to mythology or literature. Newspapers, political commentary, law, science, and everyday conversation use it to describe a seemingly small decision that creates many later problems. The phrase usually suggests that the consequences are broader and more serious than the original action appeared to be.

It is related to, but not identical with, expressions such as “opening the floodgates” or “letting the cat out of the bag.” Those phrases concern releasing something or revealing a secret, whereas “opening Pandora’s box” specifically emphasizes a cascade of harmful or troublesome consequences.