In Hesiod’s Pandora story, evils escaped into the world when the jar was opened.
Works and Days describes Pandora opening a large storage vessel and releasing the harmful troubles that had previously been kept away from human life. These afflictions explain why suffering, disease, and hardship exist among people in the poem’s mythic account.
The popular phrase “Pandora’s box” comes from later tradition, because the original Greek account refers to a jar or vessel rather than a small box. The central action remains the same in modern retellings: opening the container allows destructive forces to spread.
The story is not primarily about a treasure chest or about monsters emerging from a box. Hesiod presents the contents as evils affecting human existence. This makes the expression “open a Pandora’s box” a metaphor for triggering many unforeseen problems.