"You've got the Slime Key! Now you can open the gate in Ukuku Prairie!"
The Slime Key is a key obtainable within Link's Awakening, and is the object required to open the third dungeon, Key Cavern. In order to unlock said dungeon, Link retrieves Richard's five Golden Leaves from Kanalet Castle and returns them to him. Richard then shows Link the hidden path to Pothole Field. Link can dig up the key from where it is buried underground in this field with the Shovel.