At this point you should only have 28 Metroids left to find, so we're making good progress. Head back up to the ruins, but when you reach the long shaft leading back up, go up to the very top of the shaft and head right, you'll end up back in the large cavern with the ruins but higher up on the left wall. Use the Spider Ball to climb up the wall to find an opening further up. Go past the Metroid shell in the next room to face another Alpha Metroid.
In the bottom left corner of the room you'll find another energy refill ball, so fill up before leaving. Back in the huge cavern, climb up the wall and ceiling until you reach spikes in the roof, then drop down to land on top of the ruins. Blow open the boulder on the right and enter the ruins. Head through the next corridor to find a Metroid sitting on the ground, it'll then rise up and attack as an Alpha Metroid - you know what to do. Once it's gone you'll experience another earthquake, but there's still some exploring to do before returning to where the lava used to be.
Blow open a hole in the floor just before the Metroid's empty shell and drop down into the small room below the shell. Use the Spider Ball to climb up to the top right of the room and head through the hidden passage. Drop down the long narrow shaft ahead to find a large hidden room below. Crawl up the left wall to find a hidden tunnel, then head through there to eventually find the Wave Beam.
Go back to the large room that you dropped into and head right. The next long shaft leading down will be filled with vertical pillars of blocks, you can destroy them as you fall down. Drop down to the bottom and head left to find a corridor with two Missile Pods (you might need the High Jump Boots for the one up near the roof). Drop down the next shaft and go right to find a Missile Pod (you'll also see an Energy Tank in here, but you'll be reaching it via a different path). Go back up to the long shaft with the destructible blocks, then climb up the right wall with the Spider Ball.
Once you reach a part of the wall where the blocks are touching the wall, shoot them open and walk through the wall to find a hidden tunnel. Drop down the long shaft and blast through the door on the left with Missiles to find the High Jump Boots. You can also find an Energy Tank and a Missile Pod hidden to the left of this room. Once you've found everything head back up the long narrow shaft you dropped down through earlier and exit the ruins.
Climb up the wall to reach the top roof of the ruins. Watch out for the creature that spits out small spores, just hammer into it until it blows. Keep going right and you'll find a hole leading down, so dive on in. Once at the bottom, go left and blast through the door with Missiles. In the next room you'll find a Chozo Statue, but when you blast open the ball a creature will pop out and attack, so you'll have to fight it for the item.
It'll be bouncing around the room and often open up and fire projectiles, so active the Morphing Ball and plant bombs around so that they explode when the creature lands. After a few hits it'll explode and you'll receive the Spring Ball! Go back up the top of the ruins using the Spider Ball and head right. Down the right wall of the ruins you'll find a narrow opening past several enemies and spikes, head in there to find a Missile Pod.