After getting the Northeastern Sea Chart, you can sail to the Isle of the Dead for clues to the next pure metal.
Clues on the Isle of the Dead
When you go to the Northeastern Sea, you will find that the island on the left is blocked by a huge tornado. Sail to the northeastern island: the Isle of the Dead.
After you dock on the island, go east and enter the cave. Read the books on the tables for some clues about finding the pure metal, Aquanine. It is said to be in the Cobble Kingdom, which is near an uncharted isle. You may have found the uncharted isle when you explored the Southeastern Sea. You have to find some sort of "proof" on this Isle of the Dead to be able to find the Cobble Kingdom's isle. The Cobble Kingdom was ruled by the king and his four Knights.
Go west from the books and exit the cave there. Go west and kill the Rupee Likes from a distance, then go north and east. There are stairs leading up to a locked door. There are also Stalfos that will come to life if you get close. You can kill them with bombs. They are also vulnerable to jump attacks.
You will need to enter the graveyard of the six sages, but there is no way in right now. Go to the pyramid in the northeast and read the signs inside. It gives you a clue to finding the path from sacred tree that grows from the king's chin: West 13, North 7.
The king's chin is the southeast corner of the island. Go to the tree there and walk west across 13 tiles, then north for a distance of 7 tiles. Dig there to reveal a hole. Fall down into it.
Bomb the cracked wall that you find. Go into the room and collect all of the rupees and treasure. Read the sign, which says to face west from the king's eye and shoot an arrow. Leave the treasure room and go west. Follow the path, and be careful to avoid all of the rolling boulders.
When you are outside, you will be near the king's eye, which is full of water. Go east to find a treasure chest that contains a treasure chart. Then shoot west from the eye to hit the crystal switch. This opens the locked door in the northwest of the island. Go there.
Read each gravestone and make a note of what each one says. Some of them are hiding Stalfos, and some of them are unreadable. After you have made a note of what each gravestone says, go back to the pyramid.
Go north to find a room with the skeleton of McNey, the explorer. From there, based on what you read in the graveyard, go north, then east, then east, then north, then north. You will arrive in Brant's room. Walk up to the Sarcophagus, then tap Brant's ghost to talk to him. He will open the door for you, so go through.
Outside, go north and open the chest to get the Regal Necklace. The ghost will tell you that King Mutoh is on an island guarded by a rock wall, the Isle of Ruins. This is the island blocked by a cyclone in the Northeastern Sea. On the Isle of Ruins you will find the third knight, Bremeur.
Go back to the ship and sail to the cyclone to the west. When you approach it, it will disappear. Dock at the Isle of Ruins.