Purple liquid

A special kind of lava located on and/or within SR388 serves a much larger role in Metroid II: Return of Samus than in other games. 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 commonly believed and occasionally implied that this substance is lava. However, it is impossible to tell due to Return of Samus lacking the necessary color palette, as it plays on the Game Boy, and as such is completely in black and white or shades of green along with the Game Boy Player coloring it blue. The instruction manuals for Metroid II (both English and Japanese) only refer to it as a "dangerous liquid", but depict it as red, like lava. The Super Game Boy Player's Guide further complicates the matter by stating the presence of both acid and lava within the caverns of SR388. In Metroid Fusion, the area known as Sector 1, featured on the Biologic Space Laboratories research station and designed to be a perfect imitation of the environment found on SR388, suggests the liquid to be lava, as the sector contained replicated rooms filled with the volcanic substance. On the other hand, the area containing the said lava has a hidden passage which leads to Sector 3, which contains massive quantities of lava, yet it would not truly be an exact replica if lava was erronously present, suggesting the liquid is lava again. Octrolls thrive in this substance. A Japanese commercial depicts an Octroll emerging from an inexplicibly blue and black liquid.

Beta Acid found in Metroid: Zero Mission and Metroid Fusion is referred to as "dangerous liquid" in the latter's Nintendo strategy guide.

In Metroid: Samus Returns, the remake of Metroid II, the substance is colored purple and is still corrosive. However, instead of killing a number of Metroids to drain it from an area, Samus must upload Metroid DNA she gathers from kills into a Chozo Seal, and only then will the hazardous liquid subside.