Once you've got both missiles, go back to the main elevator and ride it up to Crateria. You'll notice that as you ride up the elevator two glowing rock faces either side of the shaft will turn to face Samus, it looks like you've been detected. Sure enough, when you go back into the "Old Tourain" area, it'll be filled with Zebesians ready to attack. Blast them all in the room and head back to the huge shaft.
Go back up the shaft while blasting the wall-hugging Zebesians, Once you reach the top head through the door in the roof. Back in the rocky cavern, you'll find more enemies crawling around the place. Go through the first door on the right up the shaft to eventually find the Map room, you can also get missile no. 1 (Crateria) through the door on the left side of the shaft, but you'll need the Bomb power-up first.
Get back up to the top of the shaft and head right. Just past the three Skree you'll notice a small opening in the floor. Activate the Morphing Ball and drop through to find another shaft leading down. Go through the door at the bottom, blast all the small enemies in the next corridor, then blow open the door at the end of the corridor with 5 missiles, just make sure you refill your missiles before you enter the room.
Once inside, grab the Bomb power-up from the statue in the corner, then wait for the Chozo Statue - also known as a Torizo - to awaken before firing at it. Refer to the bosses guide for tactics to beat it.