expression, ichidan verb
open Pandora's box
Idiomatic expression meaning to start something that will cause many unforeseen problems.
その秘密を話したら、パンドラの箱を開けることになった。
Telling that secret opened a Pandora's box.
From the Greek myth of Pandora, whose box contained all the evils of the world. The Japanese expression is a direct borrowing of the Western idiom.