In Greek mythology, the container Pandora opened was a pithos. A pithos was a large storage jar, often used for grain, oil, or wine, rather than a small hinged box.
The “box” wording comes from a Renaissance translation associated with Erasmus, who rendered the Greek word pithos as pyxis, a type of box. The mistranslation became so influential that Pandora’s box is now the standard expression in English and many other languages.
Hesiod’s Works and Days is the main ancient source for the story. In that poem, Pandora receives a jar and is warned not to open it. When she does, destructive troubles escape into the world, while hope remains inside.
The jar-versus-box distinction is one of the myth’s most common points of confusion. Modern retellings usually preserve the familiar “box” because it is part of the idiom, even though the older Greek object was a pithos.