Now that you've got the Thermal Visor and Super Missile you'll be able to access the inactive door opposite the door of the Save Room. If you scan the statue cover above the door it'll report that there's a power conduit behind the cover. Blow off the cover with a Super Missile and use the Thermal Visor to see the conduit. Shoot it with the Wave Beam and head through the powered-up door.
Cross the next tunnel while watching out for the two Pulse Bombus. In the next room use the Morph Ball to access the small opening, then boost through the open-air tunnel to avoid the Pulse Bombu's electrical bombs. Enter the next room to reach the Quarantine Cave. You'll probably remember reading info about the creature stored here from the Space Pirate labs, it's now time to meet the beast.
Drop down into the huge open space of the room and walk forward. The camera pans out as Samus walks towards the bunch of large rocks and boulders in the center of the room, but as she does the rocks come to life and arrange themselves to create a huge rock-monster creature. This thing, called Thardus, is just made up of large boulders infused with pure Phazon radiation, the source and reason for its non-inanimate activity.
It'll be a tough battle, but with tips from the bosses guide, you'll turn it back into a bunch of pebbles in no time.
Once you've crushed Thardus it'll fall apart and turn back into a lot of ordinary rocks. In the center of all the rubble the Spider Ball will appear, so it's time to claim your next Morph Ball ability - which also gives Samus' Power Suit a different look while in Morph Ball mode. You'll now have the ability to crawl around on the Spider Ball tracks like the ones riddled around the walls of the room.
Scan one of the tracks to record it into the Research section of your logbook. Use the tracks to reach the door up on the cliff on one side of the room. Boost your way through the narrow open-air tunnel in the next room to reach an elevator leading back down to Magmoor. Ahead is another track leading to a white door, so ride down the elevator. Remember way back in the Chozo Ruins when you found your second Energy Tank in the Furnace room?
There was a Spider Ball track there that you couldn't use before, but now we'll be heading back there. Once down in Magmoor, blow open the Blast Shield on the door behind you and head inside to save your game. Go through the tunnel adjacent to the elevator while watching out for the multiple flamethrowers in the roof. The next room is the large Magmoor Workstation, where you'll be greeted by three Flying Pirates.
Deal with them, once they're down you'll be free to find Energy Tank MC.02. You now have two exits to choose from, the door way up on the cliff leads to a blocked-up tunnel, so take the other door. In the following room, you'll face a large Puddle Spore sitting in a lava lake. This huge creature bears a similar resemblance to the Spore Spawn from Super Metroid, but all you need to do is fire at its core when it opens to stun it.
The creature will flip around and become a platform you can use to cross the lava. Go past the two Burrowers to reach a massive room called the Geothermal Core. Use the Puddle Spores to reach the door on the opposite side of the room and go inside. The next room contains another lava lake with a resident Magmoor. Look up above the Magmoor to spot a large stalactite in the ceiling, shoot it down to crush the Magmoor and create a stepping stone to cross the lava.
The following room houses two more lava lakes, shoot down the stalactites to cross the lava, and watch out for the Puffers and Magmoor. Note the Grapple Points attached to the ceilings, you won't be able to use them until you've found the Grapple Beam. After crossing the lava lakes you'll reach a lava tunnel leading to a familiar elevator leading back up to the Tallon Overworld. Use the Spider Ball track to safely cross the lava.
From this elevator we could go back up to the Overworld, but instead we'll keep going through Magmoor back to the elevator leading straight up to the Chozo Ruins, that way you can pick up an artifact along the way. Make your way back to the elevator, the Space Jump will make the trip easier than before.
Once you reach the Lava Lake room, blow open the large pillar in the second lake to reveal the Artifact of Nature, then jump up to claim it. Carry on to the elevator and ride up to the Chozo Ruins.