What is the modern English phrase for releasing many unexpected troubles at once?

The modern English phrase is “opening Pandora’s box.”

The expression describes starting an action that creates many unforeseen problems or releases consequences that are difficult to control. It comes from the myth in which Pandora opens a container and troubles escape into the human world.

The phrase became established in English through literary and journalistic usage. It is now used metaphorically in politics, science, technology, law, and everyday conversation. The subject need not involve deliberate wrongdoing; simply investigating a hidden issue can be described as opening Pandora’s box.

Strictly speaking, Hesiod’s earliest Greek account refers to a pithos, usually translated as a large storage jar, rather than a box. “Pandora’s box” became the standard English expression partly through later translation and retelling. The idiom’s meaning is therefore modern, while its narrative source is ancient Greek mythology.