Believe me when I say this, you're going to be here a while. This place is packed with Metroids, not to forget all the items waiting inside. You should have 12 Metroids left to find, you'll find six of them here. First, go over to the Metroid shell lying on the ground ahead and drop through it to fall down into a watery cavern. You'll find a Missile Pod over in the left corner. Once you've found it, head back up and enter the ruins (shoot the flames to get through). Once inside, climb the shaft and head left. Save your game here then move on to exit the ruins.
Out here, drop down the shaft ahead until you reach an opening in the left wall (at the base of the shaft is just a water-filled hole). Cross the next spike-filled corridor to find a room with a Gamma Metroid in it. It's time for a rare encounter, the Gamma Metroid will be flashing and it'll evolve into a Zeta Metroid, not a pretty sight. This creature's gonna be painfully annoying, as it'll be constantly trying to fly behind Samus and spit down fireballs. You'll need to hit the front of it with missiles, just jump around blasting away at it, it'll be obsessed with trying to hit Samus so you'll just have to hold out against it. Don't worry as you'll only have to face two more of these psychos.
Once it's dead head back out to the ruins and scale the cliff. Once you reach the opening leading into the ruins, duck inside and save your game, then head back out and climb up the cliff. On the left side of the ruins you'll find two doors leading inside, both paths will lead to a tall shaft containing four sealed doors. Behind each door lies either the Ice Beam, Spazer Beam, Plasma Beam or Wave Beam, so you can come back here at anytime and switch between weapons at will. You should be fine sticking with the Plasma Beam, but the Wave Beam can also help in this area.
Up the left wall of the cavern you'll find another passage leading off, this one will lead to a Gamma Metroid. Head through the spike-lined corridor to face it. Once you've crushed it head back out to the ruins and climb up to the roof. At this point you might find that you'll be low on energy after battling the last few Metroids, but don't despair as the last Energy Tank awaits nearby. Once on the roof, head right to find a hole in the ground, then jump in. This next room below will be pitch black, so it might get a bit frustrating trying to find your way around. Once Samus lands, plant a bomb to drop down further below. At the bottom, plant a bomb in the right corner of the narrow shaft and crawl right to find a Missile Pod.
Go back to the narrow shaft, then use the Spider Ball to climb up the right side, you'll eventually begin crawling along a hidden tunnel on the right. As you roll along the tunnel, hold A to jump up continuously with the Spring Ball while planting bombs, you'll eventually blow open an opening above. Use the Spider Ball to climb up to find the final Energy Tank above. Once you grab it jump up through the Metroid shell to get above the ruins again. Head right across the roof, once you reach the edge use the Space Jump to reach an opening leading right up above. Cross the next corridor to find a Gamma Metroid.
Go back to the ruins once it's dead, then drop down the right side to eventually spot a boulder blocking a small entrance in the wall on the left. Jump across from the right side with the Spring Ball and activate the Spider Ball, then blow open the boulder and head inside. Go past the Metroid shell inside to find a large room containing a Gamma Metroid. It'll become a Zeta Metroid, so be careful.
Stand on one of the two pillars to pick off clear shots at the Zeta Metroid, then once you've crushed it go up into the narrow shaft in the ceiling using the Spider Ball. In the room above, blow open a hole in the roof section on the left, then climb up around and go over to the boulder on the right of the room. Blow it open and travel through the hidden tunnel to eventually reach the Chozo Statue ahead. Shoot the ball open to reveal the last upgrade in the game - the Screw Attack.
Grab it then high-tail it out of the ruins. Once you're back outside, drop down the cliff to find a Missile refill Pod below near a Metroid shell. Fill up on missiles before heading lower. Drop down from here to reach the main entrance of the ruins. Go back over to the wall on the right from where you entered this area, then fly up to the top of the wall. Along the way you'll spot an opening on the right wall, leave it for now and continue flying up. Near the top you'll find a Missile Pod hidden in the wall, don't forget to pick it up.
Fly up through the opening at the top of the cliff to find a Gamma Metroid. Quickly jump over to a platform on the right and fight it from there. Once you've blown it apart you can find an energy refill ball on the left side of the room before leaving. Now there's only one Metroid left in this area.
Drop down the cliff below and fall down to the opening in the right wall. Travel through the spike-filled corridor to find a room filled with sand as well as the last Zeta Metroid. Try to corner it in the small opening you're standing in, as it won't be able to fly around much due to the sand. After it's dead you'll experience another well-earned earthquake, it's time to face the baddest and ugliest of Metroids.