In Hesiod’s Greek myth, Pandora opened a jar, although later tradition calls it a box. Hesiod used the ancient Greek word pithos, which generally referred to a large storage jar rather than a small container made like a modern box.
The familiar phrase “Pandora’s box” developed through later translation and literary tradition. The Renaissance humanist Desiderius Erasmus helped popularize the box wording in Latin, but this question concerns the object in Hesiod’s original Greek story. The change in container shaped the myth’s name in many modern languages.
A pithos could store grain, wine, or other goods, so the original image was a substantial household vessel. Illustrations often show Pandora lifting a lid from a box because that is the established modern convention. The jar-versus-box distinction is one of the most frequently discussed details of the legend.