What is the modern idiom for a source of unforeseen troubles opened by an action?

The modern idiom for a source of unforeseen troubles opened by an action is “Pandora’s box.”

The expression comes from the Greek myth in which Pandora opens a jar containing numerous evils or troubles. Once released, they spread into the human world. In modern English, calling something a Pandora’s box means that an apparently simple action has created a complicated set of problems that may be difficult or impossible to reverse.

The idiom is based on a translation shift. Hesiod’s original vessel was a pithos, a large storage jar, but later Latin and European traditions popularized the image of a box. The expression became established in English and other languages as a metaphor rather than as a literal reference to Greek religion.

“Achilles’ heel,” “Midas touch,” and “Gordian knot” are also myth-derived expressions, but they have different meanings. The Pandora phrase specifically emphasizes consequences released by opening or investigating something dangerous.