Hesiod said Pandora opened a pithos, a large Greek storage jar, rather than a box.
In Works and Days, Hesiod describes a container holding the troubles that later spread among humankind. The ancient Greek word pithos referred to a substantial jar used for storing goods such as grain, oil, or wine. It could be large enough to stand on the floor and was not necessarily a small household container.
The familiar expression “Pandora's box” became established after a translation history involving the Latin word pyxis, which can mean a box or small container. Renaissance readers and later artists popularized the box image, and it became the standard form in English and many other languages.
This distinction matters because the jar belongs to the earliest surviving literary version of the myth. “Pandora's box” is now the fixed idiom, even though “Pandora's jar” is closer to Hesiod's wording.