Pirate's Grotto is a large area that Mario and his team must venture through in order to find the Pirate King Cortez and receive the Sapphire Star, in the game Paper Mario: The Thousand-Year Door. Before Mario and his teammates could reach Cortez, they had to rescue a large amount of stranded Toads, along with Francesca and Frankie.
The Pirate's Grotto is a huge series of caverns and streams on the edge of Keelhaul Key. In some areas waves roll by, and debris from ships can be seen in the background. Every area shows signs that pirates once situated themselves there, including bottles, wood planks, crates, barrels, etc.