The trunk key can be found by digging in a mound of sand on the north side of the north-eastern island of Aminishi, which can be reached through the azure room inside the Temple of Aminishi. The azure room is unlocked with an azure key.
Upon digging up the key the player gets the message: "You find a worn golden key buried in the sand."
The trunk key can be used to unlock the trunk in the azure room. Doing this unlocks the A Mini Shipment achievement.
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