From the Super Mario Wiki
The Ball Room is one of the many rooms in Luigi's Mansion. It is found in Area Two.
When Luigi finds a key in the Bathroom, he uses it to open the door to the Ball Room and enter it. Inside are several Shy Guy Ghosts dancing in pairs. Luigi sucks them, causing The Floating Whirlindas to appear. Once The Floating Whirlindas are defeated, a chest with the key to the Storage Room appears.
Ghosts in the Ball Room
Ghosts in the Ball Room in the PAL Hidden Mansion
Ghosts in the Ball Room During the Blackout
Names in other languages