puzzle box
A box that can only be opened by solving a puzzle, often a traditional Japanese craft item with sliding panels or hidden mechanisms.
It took time to learn how to open the puzzle box.
Standard kanji spelling.
Mixed kana-kanji spelling; less common but still seen.
Compound of 秘密 (secret) and 箱 (box). The term likely arose from traditional Japanese puzzle boxes known as 秘密箱.