Once you open the Moon Key door in the carnival level, instead of diving down the lake under the destroyed pier, jump across to the other side using the remaining wooden pillars as platforms. While on the last pillar, destroy the crack in the wall and dive into the water behind it. Swim to the end, and you will find yourself in a little chamber with a Life Seed, Super Armor, and a block with three Skull pictures on it. When you use each picture, it will change and cycle through the graphics on each key in the game. Change the pictures on the block to Dagger, Eye, Moon to make the block retract into the ground and display the message "You're going to the secret level".