Pandora opened the container later known as Pandora’s box in Greek mythology.
The story appears most fully in Hesiod’s Works and Days, where Pandora opens a large storage jar after being warned not to do so. The Greek word pithos means a large jar or vessel, not a box. The “box” wording came from a later translation tradition and became standard in many languages.
When Pandora lifted the lid, evils escaped into the world. Hope remained inside, according to the usual reading of the myth, although scholars debate whether hope was intended as a blessing, a further affliction, or simply the final thing left in the jar. Pandora herself was fashioned by the gods and sent to humanity as part of Zeus’s response to Prometheus stealing fire.
The expression “to open Pandora’s box” now means to begin a chain of troublesome consequences. It is often used outside mythology for decisions whose effects are difficult to reverse.