In Hesiod’s Greek myth, Pandora opened a jar rather than a box. The original Greek word is pithos, a large storage vessel used for commodities such as grain, oil, or wine. It was not necessarily a small household container.
The expression “Pandora’s box” became standard because the Renaissance humanist Desiderius Erasmus translated or popularized the Latin form pyxis, meaning box, in the sixteenth century. That wording replaced the more accurate image of a large jar in much of later European culture.
This translation history explains a common mix-up: the ancient Greek version is about Pandora, her curiosity or action, and a storage jar. Paintings, books, cartoons, and modern retellings usually show a decorative box because that is the form preserved by the familiar English idiom.