It's time to leave. Leave the ruins and go back to the large cavern that previously contained lava, you'll know you're nearly there when the main theme begins playing. Drop down the shaft and crawl through the narrow tunnel on the left at the bottom. In the next cavern - yep you guessed it - there'll be more lava below, so head through the opening on the left of the room below the way you came in. Go right through the next corridor to find yourself in a new area which plays a weird theme in the background.
Descend through the large cavern through the narrow tunnels and go right at the bottom. In the next room you'll find more lava filling a large hole in the ground, so head right instead. In the next tall shaft, two openings up on the right wall will lead to two different areas, so we'll explore the upper path first. Go up to the top of the shaft and head right to eventually find a massive cavern containing more ancient ruins.
Drop down to the bottom directly below to find a Missile refill pod hidden under the sand, then climb up to the ruins. Climb up over the top of the ruins until you find a large hole filled with sand, then shoot your way down below to find a door leading left. Head inside. Climb up the small shaft ahead to find a room containing the Space Jump. Once you've found it you can now reach the upper part of the ruins.
Go back outside and shoot your way up through the sand. Fly up the wall on the right past the spikes in the wall to reach the top. Once there, go right until you find a hole in the ground, then drop down. In the large room below, shoot out the sand below to find a concealed Missile Pod, then head left. You'll find a Save station, save your game then drop down into the hole next to it. You'll fall down a long shaft with gun turrets on the walls, at the very bottom on the left you'll eventually find the Spazer Beam. Further up the shaft from the bottom head through the door on the right side.
In the next large room you'll drop down and find a Gamma Metroid, use the Space Jump to reach the floating blocks so you can fire off clear shots with Missiles. Once you've finished it off, use the Spider Ball to climb up to the top of the room, where you can blow open a section of the roof to find a Missile Pod.
Down at the very bottom of this large room, shoot open the sand in the ground to reveal a hidden tunnel leading down, so dive in. To the right you'll find a Missile Pod, in the room on the left you'll face a Gamma Metroid. Once it's cooked head left to eventually find the Plasma Beam. Once you've found it head back to the room containing the Missile Pod and go right. Use the Space Jump in the next room to clear all the spikes lining the walls up the shaft then head left at the top. Go to the end of the next corridor (watch out for the flame turrets) and crawl up the wall past the Metroid shell.
Blow open a hidden tunnel at the top and head left. In the next corridor you'll face a simple Alpha Metroid, once it's gone go into the room on the left to find an Energy Tank. Now it's time to backtrack and make your way up to the top of the ruins. Once your back on the roof, go over to the right of the ruins to find a shaft leading down. On the way down you'll spot an opening leading inside that's blocked by a boulder, use the small platforms on the right wall to jump over to the wall using the Spring Ball, then you can easily bomb open the boulder to find inside an Energy Tank and a Missile Pod.
Once you've found them head down to the bottom of the shaft, go through the opening on the right. At the bottom of the next huge shaft you'll face a Gamma Metroid. Once you've beaten it this area will be clear.