The following is an in-depth guide for the third area of Metroid II: Return of Samus.
First head all the way down and you'll eventually hit the floor. Then you'll need to go right, down a little bit and then left until you reach another shaft, where you'll need to go all the way up until you reach a left door. Keep Going to the left, killing an Alpha Metroid along the way. If you go to the left you'll notice there's a door to the next room. Use the ceiling above you to use the spider ball and go around this ledge to the right. If you keep going this way you'll get a missile expansion. Go back down and into that next room to the left. Keep heading left until right after you get to the second save spot. Go to the left until you find a spot to go down, then head to the right. Stick to the ceiling and go into it using the spider ball to get a missile expansion. Go down the bottom shaft and then head to the right to fight an alpha metroid. Right above this you can use the morph ball into the left wall on the ledge to go through and get a missile expansion. Go back out to the right ledge and go up the tube on the top and keep heading up. You'll come out to a red door that you have to destroy and go through. Bomb through the floor and go to the left. You can shoot these items to get in and get the Varia Suit. This halves the damage dealt to you and makes you go faster. Go all the way back to the last save point you've been at. Go directly left of the save point and ride on the ceiling and go to the left to go around this ledge. Once you're on top of it go to the right and you'll find another wall. Ride up this wall and get on the top of that one. Keep heading right until you find a hole in the wall. Fall down this hole and head to the left, blowing up the red door in your path. Inside, you'll fight Arachnus. The strategy to beat him is to go into your Morph Ball and go back in forth placing bombs. If you place any bombs, Arachnus will jump, so place more bombs where he will land. After you beat him, you will get the Spring Jump. This let's you jump in morph ball form without the need of bombs. Go back out to the right and go up out of the hole with the spider ball. Go to the left until you fall down, then go directly to the right and bomb through the wall. Keep heading right an fight the Alpha Metroid here. Bomb through the floor here and use your ball mode to cling to the walls and fall down a pipe. To the left if you bomb through the wall you can get the Wave Beam, but I recommend you don't grab this as the Ice Beam you currently have is more useful. Go back to the right to reach a shaft, and then go to the very bottom of this shaft and go left. In this room you can get two missile expansions, other wise head left. You'll reach another shaft, drop to the bottom and head right to get a missile expansion. Go back right to the shaft, up to the next hallway and all the way right to the next shaft. Go up and shoot through the weird spot in the wall to the right to enter another shaft. Go all the way to the bottom and shoot through the red door to get Hi-Jump Boots. This makes you jump higher. If you bomb through the floor and go left you can get an Energy Tank and a Missile Expansion. Go to the right until you reach the shaft and use the Spider Ball to reach the top of this shaft. Then head to the left to enter another area, head up and left again. Don't go all the way up as you need to cling to the ceiling so you can go through the pipe and go through to reach the higher area. Go to the left, you'll have to bomb through the floor though. Keep heading left bombing through the walls in your way until you eventually reach a shaft. You might have to fall down a bit to get here. Next, go up this shaft and take the upper right door out of it. Cling to the ceiling here to go right around this wall. Eventually, it will be a ceiling with a bunch of bumps on it. On the very first bump you can bomb through it to get a missile expansion, otherwise continue on the spider ceiling heading around the land form. Head left through the door to kill an alpha metroid. You'll need to bomb through the floor on the left side if you want to get a heal pool for your HP. Go to the right and use the spider ball to cling to the ground and go around the ground to the right. You'll reach the shaft that you were at earlier. Go to the very bottom of this shaft and go right to enter a new area. Keep heading right and you'll find the first Gamma Metroid. The same main strategies are useful here except you have to hit him with 10 missiles to kill him instead of 5. Keep heading left until you find an Alpha Metroid so you can kill it. Keep heading left and you'll have to go down a bit. Head to the left shooting through the sand to kill another Gamma Metroid. On the other side of this metroid is an HP heal pool. Finally, head all the way to the right until you kill an alpha metroid. Head to the left and you'll find a shaft that you can use to go up. Head to the right until you find another small shaft you can use to go upward. Then you head left a bit to reach another shaft. Head up until you find the firsr door to the right. Now you need to head all the way to the right for a while until you find a big shaft. You'll go past multiple save points. Go all the way to the bottom of this shaft, and then go all the way to the right where you'll notice that the lava that was there earlier is gone. Go below to enter Phase 4.