You go to the Double Cog Cavern. Open the treasure chest in the southern area. Then you can leave by the southwest door. Look west then up and use your claw shot. Return to the middle of the cog room. There is a vine patch on the northern wall. Claw shot there and get random stuff. Then leave through the door on the ledge. The next room has water in it. Dive in. Swim northward until you get to the arch. There is a bomb fish, so swim up. Go back down and blow up the bomb rock. Swim through the resulting hole. Go down further and blow up the next bomb rock. Before you enter the second hole, swim up to the tunnel in the eastern wall. Open a chest for water bombs. Dive to the bomb hole and swim through. Take out your sword because there are enemies. Go through the door at the southern end of the room. There's other stuff in this room (rupees). Look up at the claw shot medallion. Hit it and lower yourself through a hole in the floor. The boss key is in the chest there.
his in a room on the ceiling
in the main room
kill the lakebed temple boss or collect enough heart pieces
If you want to find the BOSS for the Lakebed Temple on Twilight Princess, you have to get the BIG KEY and then go to the room with the giant swiveling staircase, and just jump into the water. The BOSS DOOR is level with the water surface under the swiveling staircase. This BOSS is the coolest one so far. -Caveman1593
For the first part, use the clawshoot to pull Morpheel's eye out. For the second part, chase after him and use the clawshhot to pull yourself to his eye and attack it.
The Frog Toad is the miniboss of the Lakebed Temple. After defeating it, you get the Clawshot, which is the dungeon item.
beat the boss
It's on the map.
in the main room
You finish off the forest temple mini boss
Look on the map
The mirror of twilight is in the desert. you gain access through the mirror of twilight after defeating the boss of the desert temple.