Briny Bowl Swimming Hole
From the Super Mario Wiki
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Captain Toad starts underwater and he must find the hidden Green Warp Pipe to reach the Power Star below the course. To reach the green Warp Pipe, the player must simply enter the red Warp Pipe underwater to reach the level above, then he must drop down onto the white square-crate (located underwater) from above. Raising this box will reveal the green Warp Pipe to the Power Star. A hidden Gold Mushroom can be found under the rectangular white crate in the center of the course.
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