Mirrored Stage

The Mirrored Stage is a treasure found in Pikmin 2. It is actually a. It is located in the third sublevel in the Shower Room. It is found inside a Greater Spotted Jellyfloat. Defeating the Lesser Spotted Jellyfloats and other enemies will make it easy to retrieve it from the larger Jellyfloat.

An interesting thing to note is that the "mirrored" part doesn't provide a mirror effect, but is simply a black wall. Producing such an effect could be complicated, so Nintendo worked around it by exploiting the game's camera. In the Treasure Hoard, the camera cannot go lower than a certain level, and it is unlikely that a player will take the time to hold in the story mode to look underneath the lid.

Pikmin 2 website
"This wonderful instant dance stage that will always support the hidden singer deep in your soul. Just the thing for entertainers."