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The Island Field Office can be found on the North Side of the Ginger Island.
Until Professor Snail is rescued from the nearby cave, the office remains empty with no interactable elements. After the rescue, you can donate fossils to him and answer the Island Survey on the wall, in return for Golden Walnuts.
You may donate fossil or mummy artifacts to the office so that the skeletal or bodily remains can reconstruct the forms of four different creatures. In some cases, multiple fossils of the same kind are required.