Things You'll Need
Instructions
Using the map, locate the Mine and enter Mine Shack. The cave entrance will be in front of you. You have to get into the next level of the Mine. Do this by entering the mine by the rock on the upper right-hand side of the screen.
Enter the secondary cave. There should be a lake surrounded by treasure, and on the right hand side of the screen there should be a large door. The door is locked.
Play Puffle Rescue. Make sure to choose the Black Puffle level. Retrieve the black puffle and wait for the squid.
Follow the squid to the coral reef area. Try to catch the squid's air bubbles in order to stay alive during this trip.
Follow the steps down to the Underwater Cave. The Moss Key Pin is hidden on the right side of the screen. Retrieve the key. This will unlock the door in the Hidden Cave.