From the Super Mario Wiki, the Mario encyclopedia
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The Big Shell is a special item in Paper Mario: The Origami King. It is located in The Great Sea on the bottom of the ocean floor. Specifically, Mario must head between Club Island and Diamond Island and look for the Shy Guy. He tells him of the shell on the nearby ocean floor, which can only be collected after unlocking the submarine. Mario must dive down directly in front of the Shy Guy, locate the shell, and pull it up.
Mario can then head back to the campsite area in Whispering Woods and find the bonfire area. If he has not done so already, he must use the Magic Circle to activate the Fire Vellumental and light the firewood. The shell may then be placed onto the grill. After a while, it opens to reveal a Toad missing his vest. This Toad is revealed to be the resident of the log cabin, claiming he left in search of BBQ ingredients. The log cabin can now be entered, and Mario can open the chest inside to get Collectible Treasure #3: Ol' Grandsappy.