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Make your way back to the starting screen (to achieve this quickly, simply die anywhere on the overworld).

From the starting screen, head east, north, then east. If it's 2:35 or later you should see a stone panel with an ocarina symbol on it. If it's not 2:35, or it isn't there, you can force it to appear by using 7FFFFF23. As soon as you enter this code a waiting message appears onscreen. When it finishes you should see the ocarina tile on the ground. Stand on the tile and play the ocarina. The dungeon entrance will appear. Head on into Dungeon 6. Once inside turn the ocarina panel cheat off, as otherwise the waiting message will keep looping.