The next upgrade opens up large new sections of the game for exploration, and just to get to it you'll need to explore a new section of the Chozo Ruins.
Get to the Furnace by following the same route you took to collect the Energy Tank after getting the Morph Ball Bomb. That time you went through a tunnel near the floor into the lower part of a Morph Ball maze. This time, use the Spider Ball track to the left of the tunnel to reach a higher tunnel leading to the top half of the maze.
The floor of the top half is made of blocks that disappear very quickly after you touch them, and if you fall through you need to go back and take the Spider Ball track to get to the upper level again. You need to make a jump about half way through and that's the trickiest part. Go as far as you can go to the right, then drop a bomb right as you run into the wall. If you time it right the bomb will lift you over the wall where you can easily get over the remaining disappearing block.
Now you're in the main furnace room instead of being inside the furnace itself as before. Beetles attack when you step onto the floor, but they should be easy to deal with at this point.
There is a very interesting system of Spider Ball tracks above your head but you can't actually reach it now. You might as well get a scan of the creatures that are patrolling them though. They are similar to the other Parasite creatures you've seen except they can only be destroyed by Morph Ball Bombs.
There is a door on the far side of the room but it's the wrong color to open now, so the only way out is through a tunnel on the left as you come in. Scan the inscription above it before going through.
Crossway Access West
This tunnel turns into a regular passage.
This is a large hall and you start on a ledge over a half-pipe. Drop in, go to the far end, and half-pipe jump to the ledge on the left. There is more to do before you exit though. First scan the three inscriptions on the left. You should now have all the Chozo Lore entries but two.
Now scan the statue on the far wall to find out it's made of Cordite; that means fire a Super Missile at it. Scan the symbol that was behind it and a Spider Ball track appears on the right wall above the half-pipe. So drop back in and start swinging; the track is just about as high as you can go with the half-pipe jump, so it will take a bit of doing to reach it, but when you do activate the Spider Ball to cling to the track. Now follow the track to a Morph Ball slot, drop a bomb to reach it, and activate it.
This reveals a second Spider Ball track on the opposite side of the room, so repeat the previous performance to activate another Morph Ball slot. Drop down to the ledge below you, pushing a bit to the left so you don't roll into the half-pipe again, and enter the small hole in the wall. There is a timer on this but you should be alright as long as you don't fall off the ledge. A lift will take you up near the ceiling where you can follow the channel left to a Missile Expansion.
Now you've done everything in the room, get back to the far ledge and fire a missile to clear the door in the little passage there. There is another door here but it's the wrong color again.
Elder Hall Access
There is a swarm of Scarabs here but just roll through them.
Hall of Elders
Follow the passage toward a large hall. If you're running low on missiles the go through the tunnel on the left to find a Missile Station; just step in to get a free refill.
As you enter the main part of the room a new enemy attacks, Chozo Ghosts. These are ghosts of the Chozo, once the inhabitants of these ruins, but now driven mad by Phazon poisoning. If you have the ammo, which you do (see above), fire Super Missiles at them when they become visible. This won't be for long though and the biggest problem with fighting them is they appear out of nowhere and attack before you notice them. Try to get to a corner at the beginning of the battle to prevent them appearing behind you, and keep an eye on your radar to see where they're visible at the moment. They are immune to the Wave Beam and the other Beam types you will be getting. Also, you will be getting an item which will allow you to see them better and make fighting them much easier. One benefit the ghosts seem to have is they keep the Space Pirates away from the Chozo Ruins.
When the ghosts are gone, take a moment to look over the hall; there is a lot going on here. There is a statue of a Chozo elder in the middle of the room; scan the inscription behind it to get the penultimate Chozo Lore entry.
Now take another look at the elder, sitting there so wise and dignified. What he'd probably like is a bit of bowling, so jump into his cupped hands and switch to Morph Ball mode. The statue comes to life for a second and bowls you down a groove in the floor in front of him. There's a little ramp at the end and you fly into the air to hit the wall at the start of a Spider Ball track. If you have your wits about you, you'll cling to the track and follow it to a Morph Ball slot, now activate the slot to activate three colored disks, purple, white and red, on a ledge behind the statue.
Now scale the ledges near where you entered the room, but make an additional jump to the right. Follow the ledge to the three disks. You can't do anything with the white and red disks, but the fact that one is purple should be a clue that you need to fire the Wave Beam at it. Do so and the disk disappears revealing a Morph Ball slot. Activate the slot and something happens to the groove in front of the statue; now there's a pipe sticking up in the middle of it.
Now see what will happen when the statue bowls you again. You're taken through a hidden route to a channel on one side of the room. Follow the channel to a ledge behind a force field; it's in the corner to the left of the colored disks. Scan the panel here to lower the force field so you don't have to deal with the purple disk the next time you come through here. That's all you can do here for now so exit through the door here.
Reflecting Pool Access
Just an empty passage.
This is a large hall with a pool of water in the middle. There are Stone Toads around the edge but it looks like the really interesting things are a level up. Scan the statue at the end of the pool for a hint that it might be drained somehow. So switch to Morph Ball mode, find the plug at the bottom of the pool, and drop a bomb to break it open. The water flows out leaving a half-pipe. (Too bad about the cute little fish in there but it's for the greater good.)
Now do a half-pipe jump to the upper level. You can actually make it a bit easier by swinging in front of one of the Stone Toads up there since you can explode a bomb inside them and land on the ledge. The Toads at ground level will only get in the way so you might as will clear them away.
If you're not there already then circle around the ledges to the one on the left as you come in. Fire a missile to clear the door and go in. Note, this door and the one across from it are the last two Missile Doors in the game, so get a scan of one now if you haven't by now for some reason.
There, in a tiny room by itself, is the Ice Beam. The Ice Beam can open white doors as well as blue ones.
As a weapons it's the most powerful beam weapon yet and will often kill an enemy in one shot where the Power Beam takes three or four. Even if it doesn't kill the enemy outright, it can freeze the enemy in place and a Missile can finish the job. The downside is that the shots move very slowly and quickly moving enemies, such as Flying Pirates, will often dodge them. Fire based enemies such as the Magmoor are especially weak to the Ice Beam.
Go back, use a missile to clear the door on the other side of the room and go through.
Save Station 3
This is the last Save Station in the Chozo ruins. Now that you have what you were looking for in this excursion it's probably a good idea to save.
Expansions (Chozo Ruins)
Time to go back a bit and explore some places the Ice Beam opens up.
Hall of Elders (cont.)
Go back to the Hall of Elders; Chozo Ghosts attack again when you step into the main part of the room. Go to the ledge behind the statue and this time shoot the white disk with the Ice Beam. A new Morph Ball slot is revealed, so activate it so make another change to the groove in front of the statue. Let yourself be bowled by the statue a third time, and now you just roll into a hole in the floor.
You end up in a small hiding place under the floor. Grab the Energy Tank there, then roll back into the tunnel you came in through to be sucked back to the main part of the room.
There are two white doors in this room which are shortcuts to other rooms. There's nothing in them really but we'll cover them here to avoid leaving any holes in the map. Start by going through the white door in front of the statue.
Crossway Access South
This seems to be an empty passage with one spot where you need to switch to Morph Ball to get under a pipe. There is a tunnel hidden behind some weeds on the left as you come in, but this is also empty. So this passage just serves as an alternate route between the Crossway and Hall of Elders.
Go back to the Hall of Elders and this time take the white door to the left of the statue.
East Furnace Access
This is an empty passage connecting the Furnace with the Hall of Elders, bypassing the Crossway.
There is a final white door in this section, starting from the upper level of the Reflecting Pool, but it's a dead end for the time being. Go back to Save Station 3.
Save Station 3
You may notice a tunnel behind the station itself. Switch to Morph Ball, jump in and follow the tunnel to a door.
Transport to Tallon Overworld East
Scan the panel and step into the hologram to take the lift.
Expansions (Tallon Overworld)
Transport to Choze Ruins East
This is where the transport takes you. There is only one door so go through.
Transport Tunnel C
There are Zoomers, Geemers and Blastcaps in this passage. You need to change to Morph Ball to get through some of the tree roots passing through.
You need to switch to Morph Ball to get through the roots again. This time Venom Weed is in the way. The best way seeps to be to lay down fire to make the weeds retract, then roll through the first arch, stand up and lay down more fire, and roll the rest of the way. There is a Missile Expansion in the middle of the weeds.
Frigate Crash Site
You're now at the top of a slope over the water at the Frigate Crash Site, creating a shortcut from the Chozo Ruins to Tallon Overword. Flying Pirates attack when you start to descend.
There are two things to note here. First, because of the slope, the shortcut is one-way only. Second, you can drop from the slope down to the crashed frigate itself. Actually, using the double jump there is a way to reach the frigate through a hole in the bank, so you could have reached it earlier if there was any reason to. You have the Ice Beam now so you can enter the White Door into the ship now. While it is possible to get an Energy Tank in there now, getting to it will be difficult and time consuming. Not only that, but once you reach the tank you're at a dead end and getting back out of the ship will be even more difficult and time consuming. You're only one upgrade way from being able to explore the entire ship so it's better to hold off for now. Instead, since you're in Tallon Overworld, you might as well get back to the ship and save.