Pandora opened the jar in the Greek myth commonly known as Pandora’s box.
Hesiod’s Works and Days describes Pandora as the first woman, created by the gods and sent to Epimetheus. She was given a large storage jar and instructed not to open it. Curiosity overcame her, and when she lifted the lid, the jar’s contents escaped into the human world.
The story is often retold as though Pandora deliberately released evil, but Hesiod’s account presents a more complicated picture. The escaping contents represent sufferings, illnesses, and other hardships that had previously been absent from human life. Pandora herself is not the only cause of the disaster; Zeus arranged the entire episode as part of his punishment of humanity and Prometheus.
Modern translations usually say “box,” although the ancient Greek word refers to a jar or storage vessel. That translation difference is why the myth’s traditional title does not exactly match the earliest surviving version.