Legend of Zelda: Skyward Sword Walkthrough (Part 8) - Earth Temple Keys
Soon you'll come across a room too hot for Link to handle! Quickly run in and immediately right to fall down a slope. Stay to the right side and use the vent there to blow up towards a digging spot with a faerie inside. Then jump down onto the right path, pushing left to land on another platform. There will be two vents to your left, wait for both to be active before sailclothing across and digging up the forth key.
Continue down the slope, reaching the bottom. Turn right immediately to find a cave. Inside, use the bomb flow to bomb the blocked lava flow at the front of the cave. Once the lava drains, roll a bomb forward towards the slight slope so that it curves off towards the rock pile. Behind this you'll find a mound of dirt and the last earth temple key. Return to the top using the giant air vent you find, and enter the earth temple!