Once you beat the Fire Temple, Navi will notify you that an arctic wind is blowing from Zora's Domain. When you arrive at Zora's Domain, you will find that is completely frozen over! King Zora has been imprisoned block of red ice. Walk in behind the throne where Lord Jabu-Jabu used to be. You will find that the water is still there, but icebergs float throughout. If you haven't gotten the Farore's Wind Warp Magic, go to your right and set a bomb by the light colored boulder. If you have that, head over there anyways and pounds the other boulder to reveal a hidden hole. Use the Lens of Truth to see the enemies in the hole and near the exit, you will find a Gold Skulltula. Now go back down to the dock and make your to the east by jumping on the icebergs. Here, you'll find a Piece of Heart! Head to the west and you will find the entrance to the Ice Cavern. Let's hope you don't get frostbite!