K. Lumsy's Prison
From the Super Mario Wiki
K. Lumsy's Prison is a small, round island that the giant Kremling K. Lumsy is trapped in for most of Donkey Kong 64. The interior is much larger than the exterior but is mostly empty except for the cage holding the Kremling, although the area does contain two Golden Bananas and, through K. Lumsy, provides indirect access to parts of DK Isle and Crocodile Isle. The Golden Bananas can only be obtained by Lanky Kong, the first outside the prison with his Grape Shooter, the second inside using his OrangStand Sprint special move.
It is later seen in the ending, where K. Rool is launched straight into the prison by Funky Kong from the wreckage of the King Kruiser II, and is then beaten to a pulp by the prison's former occupant in retaliation for locking him up there earlier before being sent flying again.