In Greek mythology, Pandora opened a jar, not a box.
The earliest surviving account appears in Hesiod’s Works and Days, written in ancient Greek. Hesiod uses the word pithos, which generally referred to a large storage jar or container. The story says that Pandora lifted its lid and released the troubles that would affect humanity.
The familiar expression “Pandora’s box” developed later through translation. In the sixteenth century, the humanist Desiderius Erasmus rendered the Greek container as a Latin pyxis, meaning a small box or casket. That wording helped establish the modern image of a box, even though the ancient Greek account describes a jar.
The jar detail is an important common correction, but “Pandora’s box” remains the standard English idiom. It means an action that creates many unexpected problems.