Pandora's box
Refers to the mythological box from Greek myth; also used metaphorically for a source of unforeseen troubles.
パンドラの箱を開けてしまったようなものだ。
It's like opening Pandora's box.
Standard modern spelling.
Archaic kanji variant; 匣 is an old character for 'box'.
From Greek mythology, via Western languages. The Japanese form is a direct borrowing of the name Pandora and the word for box.