From Pikipedia, the Pikmin wiki
The Joy Receptacle (しあわせの箱?, lit.: "Box of Happiness") is a treasure found in Pikmin 2. It is actually a wrapped Christmas present. The treasure is found on the second sublevel of the Frontier Cavern and simply needs to be carried to the ship.
“Today, we ventured across the plains to reach a cold cavern I'd been meaning to spelunk. The Pikmin have had a hard time coping with the bitter cold, but they were real troopers. Somehow, they summoned the strength to carry this box, so I hope it's full of treasure.”
“Be satisfied with the decorative outer shell of this rectangle! It is more endearing since you can't see what's inside. Not being able to open it makes it all the more special ...In reality, it is empty!”
Pikmin 2 website
"This box warms the heart precisely because it can't be opened. The real happiness comes from not seeing what's inside."
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