This expression has its origins in Greek mythology. Zeus, the King of Gods, was fed up with mankind. So what's new? The Gods are always angry with us! In order to teach them a lesson, Zeus created a woman - Pandora. The King of Gods then asked all the other gods to shower gifts on her; gifts which would bring about the downfall of man! By the way, that's how Pandora got her name. Pan in Greek means all and dora means gifts. The gifts given by the gods were put in a beautiful box and Zeus presented it to Pandora warning her never to open it. If somebody gave you a beautiful box loaded with gifts and said don't ever open it, what would you do? Well, Pandora did the same thing. She opened it and saw to her horror all the evils of this world escape. According to the story only Hope remained. You may ask, if the box contained all the bad things that the gods could think of, why was Hope inside? Ask Zeus!
When you say that something is a Pandoras Box, it implies that it is a source of complication, something that will create many new and unexpected problems. The idiom commonly used these days is open a Pandora's Box.
Here are a few examples.
• The Finance Minister's reforms opened a Pandora's Box of industrial problems.
• The Vice Chancellor refused to discuss the matter as he didn't want to open a Pandora's Box.
By the way, there are different versions of this story. According to some scholars, it wasn't Pandora who opened the box, but her husband, Epismetheus, who did. Very convenient wouldn't you say? Now men can accuse women of being the cause of all their problems and vice versa.