After getting the Engagement Ring from Nana, you can sail your ship around the world, then reach Great Lakes Cavern in Australia.
Explore the Cavern
Go north across the water, then exit to the west. Avoid being in a straight line from the Douma frogs. Kill them, then go west and open the chest to get the Air Herb. The Air Herb allows you to dive where there are bubbles in the water. Go back to the previous screen and get into the water at the top and press A to dive where the bubbles are. Afterward, go east and fall down the waterfall.
In the next area, go east. Some green slimes will attack. They can cause confusion. They can also split into smaller slimes, so defeat them quickly. Go east and through the door.
In the next area, go north. There are several red Amarantes here. You can only damage them when their flower is open. Go northeast and go through the door there.
Ark will notice a shadow. Go south and stay next to the left wall. Fall down the far left side of the waterfall and you will land on a ledge where you can get the Magic Anchor. Then go to the right and go down the waterfall to the bottom. Go west to the next area.
Go north through the door. This leads to a straightforward path, where eventually you will be at the top of a ledge. Go east to find a chest that contains 753 GEMs. Then go to the bottom of this area, down the big waterfall again. Go west to the next area again.
This time, go into the water and go into the bubbles. From there, go north, then west when the path splits. Go up the steps and through the door.
Go down the stairs and go north through a narrow path. Go up the stairs and open the chest to get the GeoStaff. Go through the nearby door.
Get rid of the red Amarantes and the green Slimes in this room, then go back to the rocks on the floor and pick one up, then throw it at the dripping water on the right side of the room. Fall down the hole, then swim west, then south through the narrow passage, then southeast to find a Magirock on a small island. Then go east to reach the next area.
Go east and down the stairs, then go east and through the door. Go north past the blue Gall Fish and go through the door.
Get into the water and swim north along the right side until you find some bubbles. Press A at the bubbles to swim underwater. You will reach a chest that contains DrgnMail. Now go to the bubbles to swim underwater again.
Swim to the northwest part of the water and you will find a passage. Go into it and follow the path until you are at the bottom. Go east, then north, and you will see water. There is a spot in the waterfall that looks different from the rest. Climb up to that part and you will go through a hole into a new area.
Go north and go through the door. Then go south and get into the water. Swim north, then east, then south. When the path splits, go east and then go down the waterfall.
Now you must battle Hitoderon. There are three starfish that attack you. If a starfish goes overhead, walk under it to make it fall, but if it lands on you, it will make your controls reverse, so avoid it but hit it as it falls. The other starfish will come to the sides of the log, so just go over and hit them when they are there. Gossie (the two-headed dragon) will shoot flames at you the whole time, so try to avoid getting hit by the flames.
After the starfish are defeated, you get the HornPin. Gossie takes you to Will. Go up and talk to him. He will take you back to Nirlake. He says that he needs metal prices to go down before he can build his airplane, and you will have to talk to Lon in Yunkou to make that happen.