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Inside the tower before the first part of the area where there are platforms rising up and down and Flamers. Use the platforms to reach a tower from the left, or use Lift Cannon's lift to reach it from the right. Shatter the glass to get inside. Just watch out for the Gun Volt inside.
At the beginning of the stage, there are two paths, one leading up and another down, then forward. Follow the latter path to a dead end, and use an aimed Boomerang Cutter to nab the Sub Tank on the other side of the wall.
Found in the same large room with the Life Up. Move to the top of the platforms and head to the left past some Metall C-15s. At the end is an extra life, and the Sub Tank can be found by jumping from here and climbing up the left wall. Use the Foot Parts to break through the blocks.
After dropping down into the mines, a Mole Borer will break through a rock wall and chase X. Hang around on the wall and allow it to pass X, and explore the cubby where the Mole Borer was to find the Sub Tank.
In the same place the Life Up is in the original mode. After defeating this stage's boss and Spark Mandrill, return to the stage and use Quick Homesick to obtain it.
Go up in the area RT-55J appears in normal mode to find a Ride Armor. In the cave below, there is a small opening in the ceiling that is too high to reach. Jump from the Ride Armor to reach the opening and find the Sub Tank.
Move to the left side of Cruiziler and wait a moving platform appear. Dash jump to it and jump to the Sub Tank floating in the water. Alternatively, take the nearby Ride Armor, dash jump, and jump from the Ride Armor to reach the Sub Tank.
Go to the tower where the Sub Tank is in the original mode. Instead of a Sub Tank, there will be a Ride Armor inside. Use the Ride Armor to quickly pass by the platforms with dash jumps, and use the Ride Armor's machine gun to safely pass by the Flamers. Stand in the last platform near the Sky Claws and wait for it to move down. Punch the red blocks to open a path to the Sub Tank. The armor's explosion can also destroy the red blocks.
In the very beginning of the stage, there's a Beetron that can be tricked into ramming the breakable blocks. Instead, lure it up to the top of the area, and use its back as a platform to jump to the otherwise-unreachable cliff on the left. The Sub Tank is sitting against the wall on a ledge.
Following the area where the Life Up is, drop down into a similar section of the stage with more Jelly Seekers. Using a charged Bubble Splash, which allows X to jump higher underwater, jump up to a small spot in the middle of this area that's above water. X will just break the surface, and can actually jump on the surface if the player presses the jump button as X is about to land. With this in mind, the Sub Tank is on a ledge to the left.
After the puzzles involving the large blocks that move around and try to crush X, there is a small gap in the ceiling with a bit that sticks out, which can be climbed up. Stand on the raised platform just off screen to the left and perform an air dash coupled with a charged Speed Burner to reach it.
After passing the second Weather Crystal, stand on the first platform and air dash to the left wall. Follow this up and stay on the highest set of platforms to find the Sub Tank on the last one.
Near the beginning of the stage, there's an open area with Tomborts. Though X can use them to bypass the second level, climb up the right wall and destroy the Hamma Hamma. The Sub Tank is directly above in the far right corner, so use a Tombort to reach it. Alternatively, the player can also use the enhanced boots and perform an air dash up to it.
This Sub Tank is in plain sight above the last open area before the boss. Use the Foot Parts' (conveniently found in another part of the area) upward dash to reach the Sub Tank.
In the part of the tunnel where columns of dirt are dropped and the moving platforms, the Sub Tank is in the far right corner. The player is supposed to drop down a series of steps, and will probably spot it on the path. Simply jump to the wall and climb up to obtain it.
Beyond the room where Bit or Byte can appear, there's a long vertical passage with orbs of electricity running up and down. Head up top first and jump into a Ride Armor in the Ride Armor platform. Now return to the vertical tunnel and jump down. After reaching the bottom, the Ride Armor's mass will cause the floor beneath crumble. Follow this new path to a hidden room with the Sub Tank and some energy capsules.
At the very end of first part of the stage, (the part with the constellations) there will be a platform with the Sub Tank on it. The problem is that it's too high. Use the Falcon Armor to fly up there, or simply jump from the moving platform as it gets there. As Zero, it can be obtained by using E-Blade, or if he has the C-Sword, he can just double jump.
In plain sight in the hallway with rising and falling spiked platforms; aim a Ground Fire on the wooden plank(s) above that are blocking the way to the item.
The Blade Armor or Ultimate Armor is required. After dropping down the first time, keep heading left until finding a ledge up above. Air dash up there, then do a long air dash to the left. There will be an injured Reploid and the Sub Tank on a small ledge.
Requires Shadow Armor. A few steps forward after the Life Up, there will be a vertical shaft lined with spikes. Drop down, and head forward to find the EX Item. With care, it can also be collected by doing an air dash and landing on the safe spot.
At the end of the stage, during the fight against a squadron of Proto Rides, there are several columns. Climb one of them. Up here, several items can be found, including a Sub Tank. It must be collected before destroying all Proto Rides, as this will trigger the boss battle. It is easier to do when only one or two Proto Rides are left.
Sub Tank as parts of the Chips, developed from Rare Metals which are found in the stages. After leaving the stage and heading to Hunter Base's lab, they can be bought as parts.
While being chased by Yellow Brontes, there will be a bone structure. Jump onto the structure and dash forward to see the Sub Tank's Rare Metal in mid-air. Rewarded in the end of the stage if 6-7 challenges are cleared in their highest difficulty.
Get hit by an enemy searchlight, then use Axl's Copy Shot to obtain a Guardroid's DNA. At the far left, there will be a hidden room, with a Guardroid sleeping. Use Axl's obtained DNA to transform and salute to the guard, to open the way to the generator room. Change to X and use a charged Crystal Wall on the triangle on the floor next to the generator.
Use Zero's double jump in the first waterfall to find it. Although difficult, it can also be obtained with Axl in the first run by hovering and shooting.
After the second area with flaming enemies, there is an area that automatically scrolls down when the player jumps on the platforms. Before that, use Zero or Axl to jump up on the falling platforms to find a hidden room with the Rare Metal.
In the beginning of the stage, in the same location of the Life Up in Mega Man X. Dash and jump to the left from the highest part of the lifts to reach it.
Use Triple Rod to borrow an enemy to jump onto the ceiling passage, a small area in the wall with an Item Box. Or, when the Hittite Hottaid attacks, let it drill half of the yellow building before destroying it, and return after the mission to find a pit to the subway with this area.
Once the Pinofeel is destroyed, destroy the Pantheon Hammer in the next room and use the Chain Rod to swing over where spikes are on the sides. There will be blocks. Destroy some of the blocks but don't destroy all of them, if you do, you will fall in the spikes. Then use the Chain Rod to move the block to obtain the Sub Tank.
Use the Recoil Rod on the second (or first) Pantheon Bomber and also dash while using the Recoil Rod to obtain the Sub Tank in the top left of the area.
When seeing a button on the left of the ruin, jump on to the ledge on the right and then use the Recoil Rod charge, and also dash while doing it. There is a Petatria up there too, just attack it and it should be immobilized for a few seconds, then obtain the Sub Tank.
Use the Zero Knuckle to acquire a Card Key from Nicos Crab C; this enemy can only be encountered in the lower route, where entry requires a sunny weather to melt the snow blocking the path, or use the fire element attack to melt the snow. After the mid-boss, use it on the control panel to the right of the electrified door.
First use Model FX to break the blocks in the area being chased by the left lava, then find a hidden room at the end and press the switch inside. The switch will open the door to the room of Sub Tank, this room is located outside the exit of the area being chased by bottom lava, and use Model LX to pass through water in the room.