From the Super Mario Wiki
The Pipe Vault is a system of tunnels that leads to Yo'ster Isle in the game Super Mario RPG: Legend of the Seven Stars. The level is optional; Mario can simply bypass the Pipe Vault and move on to Moleville if he wishes. However, the Pipe Vault is the only way to get to Yo'ster Isle.
The Pipe Vault begins with a pit of lava filled with jumping Podoboos. In order to make it to the other side, Mario must traverse a series of platforms while dodging (or fighting) the Podoboos. If Mario slips and falls into the lava, he will have to go back to start and try again. After the first area, Mario will encounter pipes, treasures, puzzles, games and even a few enemies (many which are fairly weak). There is no boss in the Pipe Vault.
 Goomba Thumping Game
The Goomba Thumping Game is a special minigame run by a Mole entrepreneur from Moleville. He charges contestants ten coins to play the game, and if the contestant can reach a certain number of points, the Mole will give the contestant a special prize. Such prizes include Flower Tabs, Flower Jars, and Frog Coins.
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