The Secret Undersea Cavern

The Secret Undersea Cavern is the third mission of the Beach Bowl Galaxy in Super Mario Galaxy. The mission's objective is to make it to the Cyclone Stone. In a pre-release version of the game, this mission was known as Keepers of the Sky Gauntlet.

Layout
To reach the Cyclone Stone, the player has to destroy a large wall underwater with a Green Shell, either by throwing the shell at the wall or by ramming into it (the latter will hurt Mario). Breaking open the wall will reveal a cave with a number of crates and some bats; Ground Pounding the stump at the end will create a Launch Star that will take Mario to the Cyclone Stone.

The Cyclone Stone is a large, spiraling planet made out of stones, which are ever stationary or moving platforms. Many Thwomps and Tox Boxes move around the Cyclone Stone as well. The player has to avoid getting Mario squashed or sucked into the black hole underneath while navigating the Cyclone Stone. At the end of the Cyclone Stone is the Power Star trapped in a large crystal.