The phrase that describes releasing many previously hidden problems is “open Pandora’s box.” In ordinary English, it means to begin an action or investigation that produces a host of unforeseen difficulties.
The expression comes from the myth recorded by Hesiod, in which Pandora opens a container and troubles spread into the human world. Although the ancient object was a pithos or jar, the phrase became fixed in English as “Pandora’s box” because of Renaissance translation history.
The idiom is generally used figuratively rather than literally. It can describe a political decision, a legal dispute, a technological experiment, or a personal conversation that creates consequences far beyond the initial act. “Open the floodgates” is related in its sense of unleashing something, but it is a different idiom and does not specifically invoke Pandora’s myth.