Urashima's casket; Pandora's box
Refers specifically to the treasure box given to Urashima Tarō in the famous Japanese folktale. When opened, it releases smoke that ages him instantly. Also used metaphorically for something that brings unexpected consequences when opened.
When Urashima Tarō opened the casket, white smoke came out and he turned into an old man.
Those secret documents were like Pandora's box; opening them caused a huge mess.
closely guarded treasure; precious secret
Figurative use: something precious that one keeps hidden and rarely shows to others, like a cherished memory or a prized possession.
For my grandmother, that old photograph is like a closely guarded treasure.