Mermaid's Cave is a dungeon that you will have to visit in both time periods in order to complete. If you get stuck in one of the dungeons, go to the dungeon in the other time period to see if you make progress.
Before You Start, Get Both Mermaid Keys
See the Rolling Ridge page for an explanation of how to get both of the Mermaid Keys. The Mermaid Cave in the past requires a different key from the Mermaid Cave in the present, and you will need both keys to proceed.
Get a Gasha Seed
Start by going to the dungeon in the past. Go to the east wall and bomb the broken part of the wall. Go through.
Kill all of the Wizzrobes here (use Gale Seeds to get rid of them easily) to reveal a chest that contains a Gasha Seed.
Go east to the next room.
Get the Dungeon Map
Use Ember Seeds on the Candleheads to open the door in the northeast corner. Go through.
Swim west to reach a chest that contains the Dungeon Map.
Move the Wall
Now swim east. Go south from the torches to find solid ground. Use the Seed Shooter to light the torches in this order:
1 2 4 3
The wall will move, revealing a patch of dark water. You can't swim in this, so just go back the way that you came until you reach the entrance (or save and quit to get there more quickly).
Get the Compass
From the entrance, go north, then west. Jump over the tile in the center to reveal the Gels. Kill them. Jump over the center tile if the Gels turn invisible again. Open the chest to receive the Compass.
Get a Key
Go east, then go north until you can't go north anymore. Kill the Keese and Candleheads in this room (shoot the Candleheads with an Ember Seed) to open the east door. Go through.
Use a Scent Seed to make the Shrouded Stalfos fall into the pit. Or if you don't have Scent Seeds, jump to the platform on the right and kill the Stalfos with your sword. This makes a chest appear in the left side of the room. Open it to get a small key.
Bomb a Wall
Go west, then south, and use the key on the locked door in the west wall. Go through.
Go to the upper-left corner of the room and get rid of all but the middle pot. Push it onto the left switch. Then use the Cane of Somaria to put a block on the switch on the right. Jump over the gaps and go through the door that opened.
In the next room, kill the Wizzrobes and Wallmasters (they are both vulnerable to Gale Seeds), then use bombs to blast through the blocks. Go to the north wall and put a bomb to the right of the center of the north wall. If you placed it correctly, it will break a hole in the north wall. If you can't find it, tap the north wall with the sword to hear where the wall can be bombed.
Leave the Dungeon and go to the Present
After that, you won't be able to do anything more in this dungeon yet, so save and quit to reach the dungeon entrance, then go outside and use the Tune of Currents to go to the present. Go into the cave and use the Mermaid Key to get into the dungeon here. (If you don't have the key, read the Rolling Ridge page for instructions on how to get it.)
Get the Map
Go north from the entrance. Get past the Ropes and go north, then west. Use the Switch Hook on the stone diamond across the gap to reach a chest that contains the Dungeon Map.
Get a Second Key
Go back to the last room, then go north. Hit the crystal switch to change the floor. Go to the top right part of the room and hit the crystal switch again to get access to the door on the right. Go through.
Use a Scent Seed to make the Ropes fall into the pit. A chest will appear that contains a small key.
Get the Other Compass
Go back to the entrance and go east. Shoot the Candleheads with Ember Seeds to get rid of them. The east door will open. Go through.
There are two Giant Blade Traps. Go northeast and go through the locked door.
Use the Switch Hook to get across the gap. Go east and open the chest to get the Compass.
Get a Third Key
Go north from the entrance to reach the room with the crystal switch in the center. Hit it to make the floor reachable, then go over to the bottom left corner of the room and hit the crystal switch again to access the door on the left. Go through.
Shoot the Wallmasters and Wizzrobes with Gale Seeds to get rid of them if needed. Bomb the south wall between the two red blocks and go through.
Use the Switch Hook to get the stone diamond out of the way, then use the Switch Hook on the lower stone diamond and bomb the south wall. Go through.
Push the block down once and left twice onto the tile. Then push it right twice, up, left, down, right, up, left, down, and all the way right onto the tile. Then push it left twice, up, right, down, left, then all the way up onto the tile. This reveals a chest that contains a small key.
Get a Fourth Key
From where you got the last key, with the colorful block puzzle, go north to the room with Wizzrobes and Wallmasters. Get rid of the enemies with Gale Seeds, jump to the north part of the room, then go through the hole in the north wall to reach a room with two rotating devices. (If there is no hole in the wall, go back to the past and read the Bomb a Wall section up above.)
Use the first rotating device, then the second one. Go through the north exit. Jump down from the ledge to the right and exit south. Use the two rotating devices again and now you can use the southwest exit. Go around and get the small key from the chest.
Reach the Mini-Boss
From the chest, jump off the ledge to the right and follow the path back to the rotating device room. Use the top device and follow the path to the chest that you just opened. Jump down the ledge to the right of the chest and follow the path back to the rotating device room. Use the device, exit north, then jump off of the ledge to the right and exit south. Use the rotating device. You can go up some stairs and exit to the northeast. Go up the ledge and around to the locked door. Go through.
You will be attacked by Vire. Hit its energy orbs with your sword to get rid of them. Wait for Vire to charge toward you, then attack it. Hit it a few times like this and it will split into two bats. Wait for the bats to charge toward you and hit them.
Go through the east door.
Get the Mermaid Suit
Throw a bomb at one of the cracked blocks such that it explodes before falling into the pit. Then use the Switch Hook on the stone diamond to reach the chest. Open it to get the Mermaid Suit. Now you can swim in deep water.
Get the Boss Key
Go back to Vire's room and use the warp point. Then go east and continue until you are in the easternmost room of the dungeon that you can reach. You have to swim differently now. Press the directional pad to swim around. Pressing A will not move you around now. Go down the stairs in the dungeon's easternmost room.
Jump across the blocks, then ride the platform to the right side of the room and go up the ladder. Use the southwest exit to reach 10 rupees in a chest. Then go back to the last room, go to the northeast part of the room, then go west and unlock the key block. Go through the northwest exit. Go to the next room.
The correct lever is truly random. If Ropes fall from the ceiling, kill them before pulling the lever. Keep trying until the chest appears. Open it to get the Big Key.
Get a Ring
Save and quit to go to the dungeon entrance, then exit the dungeon and dive into the deep water just outside of the dungeon entrance (press B when in the deep water to dive). Swim to the treasure chest and open it to get a ring. Swim back to the northwest of the underwater room and press B to go back up to the surface.
Get a Fifth Key
Go outside and go to the past. Go into the dungeon. Go all the way east from the entrance to reach the deep water that you revealed when you lit the four torches. Dive into it.
Swim west. You can use any item equipped to A, so you can put your sword there to be able to attack enemies. Swim around to the stairs on the bottom right. Go down.
Swim right and go east, then go up the hole in the easternmost room. Swim against the current to use the northwest exit, then go through the exit at the top.
Kill the Ropes while avoiding the fireballs from the statues to reveal a chest that contains a small key.
Get a Sixth Key
Save and quit to go back to the dungeon entrance. Go north twice, then go west.
Use the Cane of Somaria to put a block onto the switch on the right. Kill the floating Wizzrobe with a Gale Seed if you wish, then go to the left side of the room and throw all but the middle pot, then push it onto the switch on the left. Go through the door that you opened.
In this room, you should already have bombed the north wall. If not, read the bomb a wall section above. Go to the north wall and go through the hole.
Get into the rotating device, then go around and use it from the top. Go around and use it from the top a second time. Go north and around the edge of the room to reach the deep water. Dive into the deep water.
Use the northwest exit to reach a chest that contains 30 rupees. Then go south.
Go through the northeast exit, then exit to the right. Kill or ignore the Ropes while avoiding the fireball from the statue, then exit north.
Go around and kill the ropes while avoiding the spears from the north wall. Exit to the east. Use the Switch Hook to cover the six tiles near the north wall with all six diamonds. Open the chest to receive a key.
Reach the Dungeon Boss
Go west, then go west past the spears, and go through the south exit. In the spinning room, go through the west exit. Then go south, and finally go to the southwest corner, where you first entered the underwater area, and press B to return to the surface. Unlock the west door and go through.
Unlock the key block to the left. Step on the switch until the rotating barriers are vertical. Step off of the switch and use the Cane of Somaria to hold down the button so the rotating barriers are angled toward the crystal switch. Diagonally hit the block on the left that is just below the rotating barriers with a seed using the Seed Shooter to activate the crystal switch. Go across the bridge. Open the Boss Door.
To defeat Octogon, first stand on the platform in the center of the room and get rid of the two pots. Equip the Seed Shooter. When Octogon stops and turns around, it will turn its back to you and spit a rock at you. Its back is vulnerable, so dodge the rocks and shoot seeds at it when it turns its back to you.
Eventually Octogon will be partly submerged. Get into the water and dive down. Equip your sword or another weapon to A and use it to hit Octogon repeatedly. Octogon will move toward you, so try to swim away while you do this.
After you hit Octogon for a while, it will float back up. Go back to the surface and stand on the platform again to hit Octogon with the Seed Shooter as before. Repeat this strategy until Octogon is defeated. Go back up to the surface to get the Heart Container.
Get the Bereft Peak
Go east to get the next Essence: Bereft Peak. The Maku Sprout will say that Ambi has left the palace, so you now have a chance to rescue Nayru from Ambi's Palace while Ambi is away.