Underwater Cosmic Mario Race
From the Super Mario Wiki
Underwater Cosmic Mario Race (known as Underwater Cosmic Luigi Race when playing as Luigi) is the fourth mission of the Sea Slide Galaxy in Super Mario Galaxy. Until its completion, this mission can only be played while a Cosmic Comet is in orbit.
Taking a few steps forward from where the player starts will cause Cosmic Mario/Cosmic Luigi to appear, challenging the player to a race. The clone will jump into the water and grab a Red Shell, so the player needs to do the same in order to keep up with him. Cosmic Mario will also go through two of the three Boost Rings in the water, though Cosmic Luigi will go through all three. When the racers reach the step to the tower, they need to go up and around to the back of the tower to find the Power Star. If the clone reaches the Power Star first, the player will have to start the race over.
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