The Tuna Desert is accessible by interacting with a fish stuck in the ground in Pancake World.
Features
The aptly-named Tuna Desert is filled with numerous small pyramids, giant tuna, and tuna heads/tails, as well as tuna-headed NPCs with sandy bodies that bark when interacted with.
A vase can be found here that, when interacted with, takes Urotsuki to a small tropical island with palm trees, where a man in a turban can be seen residing on the smaller, isolated part of the island. Traveling up north, you can find a small building with a construction sign blocking the entrance.