Skull Key is a wallet item used to unlock access to the Skull Cavern. The cavern entrance is located in the northwest corner of the Calico Desert, which can be accessed by riding the Bus after it has been repaired. The Skull Key is found on the final floor of The Mines (floor 120) in a reward chest, and can then be viewed in the Special Items & Powers tab of the player's inventory screen.
Acquiring the Skull Key also allows the player to play on the Junimo Kart arcade machine in The Stardrop Saloon.