Where do I find the 5 Pieces of Key, required to enter the Earth Temple?

I'm now ready to enter the Earth Temple, but it is locked, and the key is in 5 pieces.

I've already found 3 Pieces of Key, but I'm still missing 2.

I've tried dowsing for them, but I can't seem to find them.

Solution 1:

The First piece is located just to the left of the entrance to the Earth Temple. It's buried in a soft spot of soil, so you'll need to use the Digging Mitts to dig it up.

The Second Piece is located under the wooden watch tower down a path at the left of the temple. It's at the bottom of a hill of sand. You'll need to roll a bomb flower under it in order to have it fall over, so you can access the soft soil containing the piece below.

The Third Piece is located behind a boulder, about halfway up the giant sand hill in front of the temple (if my memory serves me correctly, it's how you got up to the earth temple to begin with). You'll need to roll a bomb flower in front of the boulder so it will destroy it, revealing a spot of soft soil containing the piece.

The Fourth Piece is a bit trickier to get to. You need to take the path to the right of the temple, and follow it along until you reach an area where it's too hot to stay long. Fi will give you a warning when you reach it, saying you can only stay here for a short while before dying. There are two ways out of this area, and you need to run for the correct one. Use your dowsing ability to find it. once you come out, you will be at the top of another giant hill.

Take the right path down this hill, and you'll eventually come across a ledge on the left wall of this path. You need to get up there, and then use the air geyser there to get across to the other side of the ledge, which will contain a soft spoil spot with the key piece.

For the Final Piece just proceed down the hill. Jump over the first platform, near the right, and then take another right. There will be a small cave containing a pool of lava, a boulder, a bomb flower and a sand hill. The key piece is located behind the boulder.

In order to break the boulder, you need to use a bomb flower rock, just outside the entrance to the cave, near the left. It will have lava leaking from it. Once you blow this up, the lava level will lower and you'll be able to roll a bomb flower over to the boulder so that it will be destroyed, opening the path to the bomb flower.

Finally, here's a video showing someone gathering all the pieces:

(Images taken from zeldadungeon.net)

Solution 2:

According to this MyCheats page:

The Earth Temple's entrance is locked. You'll need five Pieces of Key to open it. Here are their locations:

1) Just west of the main door in a dig spot. Inside is a Piece of Key.

2) Just east of the main door is a gilded staircase. There are some Bomb Flowers on the edge of the slope. Use these to bomb the wall on the east side of the slope. Dash to the bombed wall and dig for the Piece of Key.

3) One is further west of the door. Slide down a slope to the southwest of the main door and dig there for the Piece of Key. Run back up the slope to return to the door area.

4) To the northeast of the locked door is a precipice overlooking a wall of vines. You have to drop down and scoot along a ledge to the left of the broken bridge here towards a Heart Plant. Follow this path up and then use the ledge to jump to a higher ledge. From here, jump across to the vine wall. Quickly move right and, if you need it, grab the Stamina Fruit. At the top, hit the button to extend the bridge.
There's an intense-heat area here which will light you on fire -- but you need to pass through here, despite the risk. Head in and run to the right and down the slide. You can get some goodies on this slide, but you'll have to return to the top for the Piece of Key.

Goddess Cube - Take the left side and Aim for the left platform with a Dig Spot on it. Below this is another platform with a Goddess Cube on it. Note that the platform on the right as you slide down has a Pink Fairy on it in a Dig Spot.

At the bottom, you can bomb a wall to create a shortcut back to the south. Take the large geyser back up to the top of the slide here. This time, take the right path for a landing far below. A Pink Fairy can be found in the digs spot on the platform you come to first.

Two geysers are just off the edge of this platform. You need to time this jump perfectly. Wait for the far geyser to spout before jumping. This will get you to the next Piece of Key in a dig spot on the platform.

5) At the bottom of the long slide is a tunnel leading north to a cave with a sandy incline and a lava pool.

At one end is a plug in the wall. You can toss a bomb at this plug. To do this, run up above the plug with a bomb flower and toss it down. Now, with the lava filled in, you can toss another bomb at a bomb-able wall. BOWL this bomb at the wall. Inside is the final Piece of Key.
Head to the dungeon entrance with the five Key Pieces and the door will open.