Instructions
Go to the top of the mountain in the hub world after completing the sixth temple. Use the magic mirror item on the medallion spot and then warp to the top of the big rock.
Walk straight (past the other temples and the bridge) until you see a large rock with three pegs on it. Push down the peg on the right first, followed by the center and then left peg. A transporter will appear. Walk through it to enter Dark World.
Step on the tile with the "quake" symbol on it and then use the corresponding spell. The head of the turtle rock statue will turn into a staircase. You can now proceed into the Turtle Rock temple.