Wave Beam Search

Walkthrough: With the Varia Suit, you can finally enter the lava rooms. You can't do anything in the nearest Lava Room to where you are at, so travel back to the main room. Enter the biggest of lava rooms with the door to the left, and make sure not to get carried away by the flow. Also, avoid the fire attacks and lava, as they still cause damage to Samus. Start making your way towards the room with the next Missile Expansion.

There will be two exits in the lava room, shown by another top door and bottom door. The top door is being presently blocked by something, so you'll have to go instead the bottom door. Here, you'll find the next Metroid; however, this is one of the Gamma Metroids, a Metroids even stronger than an Alpha Metroid. After destroying the Gamma Metroid, head into the next passageway on the right.

Wait patiently for the wall-fire to briefly stop, and then shoot a missile at the weak point in the wall and then jump to it. Use your Spider Ball on the ceiling, and then break the next weak block with a Bomb. This will reveal another hallway, one that will take you to a room with an Ammo Recharge Station. There will also be another door. Go inside to find and obtain the Wave Beam. After being equipped with it, return to the previous room and his the purple substance blocking the path to the door near it. This will bring you back to the main room.

Missile Expansion: After getting the Varia Suit, enter the big lava room. Above the wall-fire are two blocks that need to be destroyed. Spider Ball up and Bomb them to reveal the next Expansion.