In classical mythology, a box that Zeus gave to Pandora, the first woman, with strict instructions that she not open it. Pandora's curiosity soon got the better of her, and she opened the box. All the evils and miseries of the world flew out to afflict mankind.
To “open a Pandora's box” is to create an uncontrollable situation that will cause great grief.
A source of unforeseen trouble, as in Revising the tax code is opening a Pandora's box. This equivalent for the modern can of worms comes from the Greek legend in which Pandora, entrusted with a box containing the world's ills, is overcome by curiosity and opens it, thereby releasing them. [Late 1500s]