Purple liquid

Unknown Substance 1 is a highly corrosive liquid that exists on SR388. It blocks off the areas on the planet, and decreases during an earthquake, which is caused when Samus Aran kills a certain amount of Metroids in the area.

It is common misconception that this substance is a sort of Acid or Lava. However, it is impossible to tell due to Metroid II: Return of Samus lacking the necessary color palette, as it plays on the Game Boy, and as such is completely in black and white. The instruction manual for Metroid II only refers to it as a "dangerous liquid", debunking this theory.

A possible explanation for what the substance is stems from pits underneath discarded Metroid shells, which may be caused by a corrosive byproduct excrement that is produced during the Metroid molting process. Thus, if this were true, then the "dangerous liquid" might be excrement created by Metroids.

Helping to bolster the theory is the presence of a highly corrosive blue-green liquid in Mother Brain's chamber after Tourian is destroyed in Metroid: Zero Mission, replacing the lava that was there before. It is possible that this is also Metroid excrement that was extracted by the Space Pirates for unknown purposes, and flooded Mother Brain's chamber after Tourian was destroyed. It is severely dangerous to step into even if Samus is wearing the Gravity Suit. This substance is not present in Super Metroid, which could suggest that it dried up or was drained by the Pirates.

The same substance is also present aboard the Biologic Space Laboratories research station, in two corridors in the Tourian-like area of Sector 1, one of them being the room before Ridley-X's chamber. Like in Zero Mission, the substance is harmful even to the Gravity Suit, even though Samus had been infused with DNA from the Baby, technically making her a Metroid. It is possible that the scientists at the BSL station were extracting the excrement for the same purposes as the Space Pirates, if the above theory is true.

However, other areas of Sector 1, which is a replica of SR388's environment made so that the Metroids evolve in their natural life cycle, contain lava, debunking the above theories.

This substance, being corrosive and green, may be yet another reference to the Alien film franchise, as the Xenomorphs in the franchise bled corrosive green blood.