After you defeat Slime Eyes in the Key Cavern, you are told to seek the next key in the Yarna Desert.
Reach the Desert
East of the village, there is a tree with a big beehive on it. If you go there now, you find Tarin standing in front of it. Talk to him and let him borrow the stick. Then when the honeycomb falls, take it.
Go to the southeastern part of Ukuku Prairie, where there is a sign that mentions Animal Village and Martha's Bay. Go down the stairs in this screen (it might be hidden under a bush). In the underground area, get past the Piranhas and then use the Pegasus Boots to dash through the crystals in the eastern room. Afterward, go up the stairs.
Go south until you reach a sign that says to go east to the Animal Village. Go east to get to the village.
Go into the southeast house and talk to the bear. Give him the honeycomb to get a pineapple. Then he will tell you to go get Marin, who can wake up the walrus who is blocking the path to Yarna Desert.
If you go to the weather vane in the village, the boy there says that Marin is at the beach. You can find her at the easternmost part of the beach, southeast of the monkey in the tree. Talk to her, and agree to stay. When she asks if you're listening, say yes.
Afterward, Marin follows you. Go back to the Animal Village and go south from it to reach the walrus. Walk up to the walrus and say Yes. Marin will wake him up. After Marin leaves, go east into the desert.
Get the Key
In the northeast of the desert, you will find a bird statue and a cave. Go west from that area to find an area of quicksand. Lanmola attacks. Hit its head when it emerges from the sand. If you get stuck in the middle of the quicksand, you will fall into a cave. If this happens, exit the cave and go west to fight Lanmola again.
After you defeat Lanmola, you get the Angler's Key.
When you leave the desert, the Owl tells you to go to the waterfall in Tal Tal Heights and fall from above to reach the dungeon.
Unlock the Next Dungeon
To unlock the next dungeon, go east from Goponga Swamp. Eventually you will reach an area with two Pig Warriors and a rock. Lift the rock and keep going east. You will find the keyhole. Push on the keyhole to open the dungeon. However, to get there, you have to climb the mountain.
Reach the Next Dungeon
Go west from the keyhole until you reach some stairs going up the mountain. Go up those stairs. Lift a rock to enter the cave. Go to the east side of the cave and push the first rock upward into the hole, then hit the northern crystal with your sword. There is a rock that was next to the crystal that you just broke. Push that rock to the right, then push the one below you down. Go down the stairs.
Go through this cave, being careful to avoid the green Zols hiding in the floor. Go up the stairs at the end, then go east. Use the Pegasus Boots to dash through the crystals to the east. In the next room, go outside.
Go east, then walk east in the shallow water to reach a cave. Go into the cave. Go through the cave, avoiding the green Zols that are hiding in the floor. Go up the stairs in the second room. Ignore the chest up above, because it contains a green Zol. Go south until you're outside. Go southwest to find Papahl. Talk to him and agree to give him the pineapple. If you don't have it, you have to find the tree with the beehive east of the village, and talk to Tarin to give him the stick. Take the beehive, then give it to the bear in Animal Village to get the pineapple.
When you give Papahl the pineapple, he gives you a Hibiscus. Later you can go to the Animal Village and give the Hibiscus to the goat who is writing a letter. Then take the letter to Mr. Write, who is north of the Mysterious Forest. He gives you a broom. Give the broom to the old lady in front of Ulrira's house. She gives you a fish hook.
After rescuing Papahl, go north and then jump down the ledge. Walk east through the shallow water. If you used the Angler's Key on the keyhole, you will find an area where there is no water, and there is a place where you can jump down. So jump down. You will land in front of the dungeon entrance. There are stairs here that will allow you to go back to the area where you used the key, but it's a one-way shortcut, so if you want to go back to the dungeon, you have to climb the mountain again.
When you're ready, go into the dungeon entrance. It's Angler's Tunnel.