In Hesiod’s Pandora story, human ills escaped from the jar after Pandora lifted its lid.
Works and Days describes the container as holding destructive conditions that had previously been kept away from humanity. Once the lid was removed, these evils flew out and spread among people. The poem lists suffering associated with disease, toil, and other hardships rather than presenting the contents as a single named monster.
Modern retellings often summarize the contents as “all the evils of the world.” That wording is useful, but “human ills” is closer to the range of troubles described in translations of Hesiod. The story is part of a Greek explanation for why human life includes pain and labor.
The jar’s release is also why the phrase “open Pandora’s box” means to begin a process that creates many unforeseen problems. In the ancient account, the crucial action is opening the vessel, not breaking it or smashing it.