Wario Land 3: Part 60: Beneath the Waves (Gray Chest)
5At this point, I've gotten 4 out of the 5 Music Boxes! Now to get one more!
The 4th Music Box unlocked another aquatic level called Beneath the Waves. This refers to the wavy currents that you'll have to swim through to reach the doorway in the middle of them.
But first, head to the dry doorway on the right. In the next room, the Gray Key is hiding withing the pillars above you. You need to use an enemy to High Jump bounce off of them between the correct pillars to reach the Gray Key.
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Now, for the doorway in the middle of the currents, use more zipline pulleys in a way that allows you to roll your way through the path towards the Gray Chest.
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Beneath the Waves' Gray Chest Treasure is... an Explosive Plunger Box!
Detonating this causes come Bomb Blocks in some levels to blow up, allowing new paths to be taken in Cave of Flames, The Stagnant Swamp, and The Colossal Hole.