From Zelda Wiki, the Zelda encyclopedia
The Slime Key is required to gain entry into the Key Cavern in Link's Awakening. To obtain the key, Link must travel to Kanalet Castle to retrieve Five Golden Leaves for King Richard. In return, King Richard will grant Link access to his dangerous back garden where an owl statue is located. When Link uses the shovel to dig under the statue, the Slime Key will appear. The Key has a close resemblance to the Slime Eye, the final boss of the Key Cavern, and is supposedly named after it.