The Secret of Buoy Base
When Mario initially dives into the surrounding pool of water, he can find a caged green pipe and open it by luring one of the Torpedo Teds into it. The pipe takes Mario to the outside of the planet, where Mario must lure Bullet Bills into a cage that contains the Green Power Star. Falling off the platforms underneath the planet results in Mario ending up back in Buoy Base's water pool.
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