After Poison Moth's Lair, the Maku Tree says to go to Sunken City and look for the next essence there.
Continue the Trading Sequence
Now that you have the Roc's Feather, you can continue the trading sequence. Go to Subrosia and head west from the Subrosia Hot Springs. Find the house with a chef in it. Give the iron pot to the chef to receive some lava soup.
Reach Sunken City
Go east from Poison Moth's Lair, then up the stairs and east again. Go south, until you can't go south anymore. Go east, then north to the Pegasus Seed tree. Go east from there, then south. Keep going eastward until you are in the area where Ricky helped you. Go east until you reach a screen with a hollow log across a hole in the ground. Jump over the hole and go through the log. Go east from there.
The next area will be different depending on which animal you befriended. If you have Ricky, it's Natzu Prairie. If you have Moosh, it's Natzu Wasteland. If you have Dimitri, it's Natzu River.
If you have Ricky
Hit the switch to extend the bridge. Go up the stairs and jump up the ledge, then go east as far as you can go. Jump down the ledge to the south. Continue south and use tornado punches to get rid of the bushes. Keep going south until you reach a cliff that you can jump up. Follow the path around until you drop down another ledge. Go east, then north, and follow the path around. You'll pass the Great Moblin's Keep. Go east to access Sunken City.
If you have Moosh
Hit the switch to extend the bridge. Go east and float over the holes. When you can, go north, then east from there. Continue east and float over the holes, then go south. You'll pass the Great Moblin's Keep. Go east to access Sunken City.
If you have Dimitri
Go east. Keep going northeast utnil you reach dry land near the purple castle. Go east and jump over the holes and you pass the Great Moblin's Keep. Go east to access Sunken City.
Use the Roc's Feather to jump to shallow water and make your way around the city. On the east side of the city is a Gale Seed tree. Gale Seeds warp you to any seed tree that you have seen.
If you don't have Dimitri, go west to find some kids harassing Dimitri. Talk to the middle kid and give him all of your Bombs. You can then ride around on Dimitri. If you do have Dimitri, you should call him now.
Go north from the Gale Seeds tree and go up the waterfall. Ignore the house up here because you can't get into it right now. Instead, go west from the house and go up the leftmost waterfall. Push the four statues so they form a square around you and use a spin attack. Then jump over the water to reach the chest and get the Master's Plaque.
Take Dimitri outside and go down the waterfall, then go up the rightmost waterfall. In the cave, get 50 rupees from the west side of the room, then talk to the Master Diver. Seeing the Master's Plaque, he gives you the Flippers. He tells you his secret route to Mt. Cucco. He tells you to find a diving hole among the reefs in down and press B to dive into it.
Go to Mt. Cucco
Go to the northwest of Sunken City. Swim into the water east of Ingo's House and go as far north as you can go, then dive. Swim around killing Cheep Cheeps and go west. At the end, go up through the hole in the ceiling.
Go up the vines. Go north, east, north, and up a long flight of stairs. Jump off of the rightmost ledge to end up in Subrosia.
Get your Items Back
Jump over the lava and a couple of Subrosians will steal your Roc's Feather and give you Fool's Ore in exchange. Go west and into the weird-looking house. Follow the Subrosians west and outside. They will move around this area much like the Subrosian girl you followed before you first entered Subrosia. Avoid letting these two Subrosians see you. You can't exit the screen until they're gone, so just wait for them to leave, not letting them see you.
Eventually they'll both stand next to a red square, then leave. Dig at that square to get your feather back.
Get the Power of Spring
Go back through the weird-looking house and into Subrosia. Go east from there to where you can see some stairs across the lava. Use the Roc's Feather to jump there and go down the stairs.
Jump along the moving platforms and exit to the right. Jump across the next moving platform, but go past the brown ladder and go down the ladder on the right to get a gasha seed. Go back up, and go up the brown ladder.
You arrive in the Temple of Seasons, next to the Spring tower. Go into the door and use the Roc's Feather to jump to the stairs at the top middle of the room. Continue until you reach the altar. Walk up to it to have the power of Spring added to the Rod of Seasons. It makes rock-hard buds bloom. You might remember seeing one in Horon Village, and if you were in the village in springtime, you saw a kid using the bud to jump up to the top of the ledge.
Give Moosh the Spring Banana
Go back to the vortex that took you from Mt. Cucco to Subrosia. Use the vortex to go back to Mt. Cucco. Jump down off of the ledge and use the tree stump to turn it into spring.
Go east to find Moosh. He wants Spring Bananas, but it has to be springtime to get them. If you didn't already turn it to spring, go west, use the tree stump, and go east and then north.
Use the Power Bracelet to pick up the Flying Blue Cucco. Tap the A button to float as high in the air as you can go, and keep tapping it until the Cucco drops you off at the top of the screen. If you didn't tap the button enough, the Cucco drops you off before the ledge and you have to try again. From the ledge, jump down to the cave entrance and go in.
Use Pegasus Seeds to help you get onto the fast platform, and quickly run to the right when the platform starts to go north so that you end up where the switch is. Step on it, then get back onto the moving platform to go around and get the ring that falls.
Go back outside and jump off of the ledge, then go north and use the Flying Blue Cucco again. This time instead of jumping down to the ledge with the cave, go north and go into the cave here. Use the Roc's Feather to jump to the small platforms on the right side of the room. Jump to the platforms against the north wall and then jump west to the stairs. Go up. Swing your sword at the tree to get the banana. Then go down the ledge, go south and jump down the ledge. Go south to Moosh. Talk to him to give him the banana.
Climb the Mountain
With Moosh, go west. There is a business scrub who hides when you go near. Stand back and raise your shield. The business scrub will come up after a moment and spit seeds. Bounce them back at the scrub until he stays above ground. Talk to him and he mentions that even if you have a key, you still need a keyhole. He says that the keyhole is on the summit. To the west of this scrub, you find a similar one. If you hit this one with his own seeds, he says that the key is in the valley in the west. You can use the flower to the right if you wish, but there's nothing interesting up there yet.
Optional: Continue the Trading Sequence
Instead go west. You end up in snowy Goron Mountain. You can go to the west to explore, but it's dangerous and there isn't much to find there anyway. If you have Lava Soup, though, go all the way west and go into the cave. Go to the back of the cave to find stairs, then in the upstairs area jump over the lava to go outside. Go up the stairs to find Biggoron. Give him the Lava Soup. He gives you the Goron Vase. The person to give it to is Ingo in Sunken City. He has the house in the screen where you dove underwater to find the secret passage to Mt. Cucco. When you give the Vase to Ingo, he gives you Fish. You can give this to the guy east of Impa's house in exchange for the Megaphone.
Bigger Ring Box
You can also get a bigger ring box on Goron Mountain. Go back into the cave with the gorons in it, and go back outside. Go east one screen and go into the cave. Go around to the upper-right area and go up the stairs. Once outside, go east and up the stairs, then west and into the cave. Go north, being careful of hidden spiny beetles in the bushes. Outside, go east. Be careful of the Blue Lynels. They require a lot of damage to defeat, and they shoot sword beams that damage you by a lot. Go around the path and go west. Cross the bridge and jump off of the ledge. Go into the cave and talk to the red goron to receive a bigger ring box. Now you can carry more rings, although you can only equip one of them at a time.
Now go back east to reach the first snowy screen. If Moosh isn't with you anymore, go back to where you gave him the Spring Banana. Go north from the first snowy screen and use Moosh to float over the holes and reach the key. Then go south and east back to Mt. Cucco. If it's not spring, go east to find the tree stump and use the Rod of Seasons to turn it into spring.
Go west from the stump until you reach the screen with the flower and the business scrub that spits seeds. Get off of Moosh and walk onto the flower. It will spit you up to the ledge. Go north, then east until you reach the tree stump, and use the tree stump to change it to winter.
Go all the way west, up the stairs, and jump onto the snow. Go east and into the cave. Use the Roc's Feather to jump over the gap at the top of the room. Go west from there. Use the Roc's Feather to reach the stairs at the top-left corner of the area. Then go around the right side of the room to reach the stairs below.
You should now be on top of the mountain. Go east. Use the Power Bracelet to pick up the Cucco. Tap A to stay as high up in the air as possible. The Cucco will drop you off on the ledge on the right. Go east. Push on the keyhole to make the waterfall disappear.
Go west and fall off of the ledge. Go east and stand on the tree stump and use the Rod of Seasons to turn it to Summer. Then climb up the vines and go into Dancing Dragon Dungeon.