Where to find
It can be found on the main island, on the teak forest map in the area where the Strong Screw is located. It is
under the rock on the island to the east of the Strong Screw island, which is only accessible at night.
The ancient key has a small chance of appearing after the player punches the rock before you obtain the Treasure Box. After you obtain the Treasure Box (which is after you reach 100% completion of The Book) and learn the recipe for the Mysterious Engine, the key will appear under the rock.