In Hesiod’s Works and Days, Pandora’s jar released evils into the human world.
Hesiod describes the container as a large storage vessel, not a small box. When Pandora opens it, destructive troubles fly out and spread among human beings. The story explains why suffering, disease, and other hardships exist in the world, while one remaining thing stays inside the vessel.
The popular expression “Pandora’s box” developed later because translators and retellers connected the Greek word pithos with a box-like container. The ancient image is therefore better pictured as a jar or large earthenware storage vessel.
Retellings sometimes say that the jar released one specific evil, but Hesiod’s account describes evils collectively. Fire is also a frequent mix-up because Prometheus’s theft of fire is part of the surrounding mythic sequence. Pandora’s opening of the jar is a separate event with a different consequence.