The story of the “Pandora's box” comes from ancient Greek mythology. According to legend, Pandora was the first woman on Earth, created by Zeus. He gave her a box (or, more commonly, a jar) that was not to be opened. But over time, she could not resist the temptation and opened it, releasing all sorts of evils and troubles into the world. Interestingly, only hope remained inside.
Thus, “Pandora's box” is an idiom that means a situation where a careless action can lead to a large number of troubles or problems. So, when we say “to open Pandora's box,” it is like hinting at the beginning of something undesirable and unpredictable.
Here are a few examples of using this expression: “Reading the old letters was a mistake — I opened the Pandora's box of memories and pain that I had almost forgotten.”
“Genetic editing of embryos is a Pandora's box: at first glance — a breakthrough, but the consequences can be uncontrollable.”
“The introduction of this reform without preparation opened a real Pandora's box — a destabilized economy, mass protests, and a loss of trust in the authorities.”
“When he started talking about their past, he didn’t realize he had opened Pandora's box — now they both are in pain.”