Guppy and the Underground Lake
From the Super Mario Wiki, the Mario encyclopedia
The player must make their way down to the bottom of the underwater cave. There, they will find Guppy waiting on a star-shaped rock, and approaching him will have him challenge the player to swim through eight rings. All eight rings are placed either within a group of enemies or an obstacle. The player can use Green Shells to move through all the rings before they disappear, and failing to move through a ring will cause the player to fail the challenge. Swimming through all eight rings before they disappear will have Guppy give up a Power Star.
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