Purple liquid

Purple liquid, also known as hazardous liquid is a corrosive substance in Metroid II: Return of Samus and Metroid: Samus Returns. It blocks off areas on planet SR388 and must be drained by defeating all Metroids in the current area in order to continue exploring. The titular creatures do not live in rooms where the liquid is present.

Metroid II: Return of Samus
In Return of Samus, the liquid decreases during an earthquake, the latter which is triggered when Samus Aran kills a certain amount of Metroids in an area. The game offers no explanation on how and why their deaths trigger or coincide with SR388's earthquakes. The liquid is slightly opaque compared to other liquids seen in the game.

Metroid: Samus Returns
In the remake of Metroid II, Samus must kill all of the Metroids in a given area and collect their DNA in order to upload it into a Chozo Seal that drains the liquid from the corresponding location. The substance is corrosive to the point of leaving a dark purple taint on the surfaces of tunnels the liquid used to occupy. It is very harmful to Samus even in her Gravity Suit, yet it seemingly poses no threat to all native life on SR388 with the exception of Metroids. Indeed, Gawrons are seen emerging from Air Holes inside the liquid, Halzyns can land inside the liquid and emerge unharmed, and many animals such as Motos and Hornoads are seen roaming the very caverns where the hazardous liquid was once present mere moments ago. However, enemies do not appear inside the depths of the liquid, so the latter's presence can alternatively be explained by the creatures simply exploring their newly accessible environment every time Samus removes the purple substance, similar to how they suddenly repopulate rooms after she kills the resident Metroids.

Metroid: Samus Returns Official Guide and the 9th Chozo Memory reveal that the liquid was used by the Chozo against their out-of-control creations, the Metroids. The purple substance is seemingly harmful to Metroids as they tend to be found in far proximity from it, successfully keeping them at bay deep underground within the main areas. Its harmful effect on non-native lifeforms is also of intentional design, as it is meant to discourage and/or prevent off-world visitors from exploring the planet. Thus, the substance not only protects visitors from certain death against Metroids, but it also keeps the species from falling into malevolent hands.

Unfortunately, the implementation of the substance was ultimately flawed. Despite the Chozo flooding their own cities and massive portions of the planet with the liquid, an untold number of Larva Metroids managed to reach the Surface of SR388 regardless, thus allowing the species' eventual discovery by the Planet Reclamation Division of the Galactic Federation. This led to the events of Metroid/Metroid: Zero Mission and the entire history of the creatures spreading across the cosmos.

Debate
Prior to the release of Samus Returns, it was commonly believed and occasionally implied that this substance was either lava or acid. However, this was 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 referred to it as a "dangerous liquid", but depicted 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.

Metroid Fusion seemed to support the idea that the substance was lava, given the fact that there was a room in the SR388 replication sector of the B.S.L filled with it. However, it was revealed in Samus Returns that both lava and purple liquid existed on the planet and were not the same.

Octrolls thrive in this substance. A Japanese commercial depicts an Octroll emerging from an inexplicibly blue and black liquid. They are not present in Samus Returns.

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

Metroid: Samus Returns manual (Q&A)
"Q: I'm taking damage when I jump into water... A: It may look like water at a glance, but it is actually a hazardous liquid that will cause damage, so you'll need to watch where you jump."

Metroid: Samus Returns Official Guide

 * Inhabitants & Eco Systems of SR388 (p. 24)
 * PURPLE LIQUID
 * "This liquid doesn't deal quite as much damage as lava, but it causes a lot of problems if you stay in it too long. It serves as a gate between areas, its purpose to keep the Metroids from reproducing, escaping, or being taken by those who are up to no good &mdash; like Proteus Ridley, Mother Brain, and the rest of the Space Pirates."

Trivia

 * The lack of purple liquid between Area 8 and the Surface of SR388 is potentially the reason why Larva Metroids were able to reach the latter area after the release of the purple liquid.
 * However, the Chozo Memories do not offer details on the amount of time it took for the bird race to create both the purple liquid and Chozo Seals, leading to the possibility that Metroids may have simply escaped SR388's cave systems before the flooding took place.
 * The Purple liquid was introduced by the Chozo to hinder Metroids, which may explain why the purple liquid is not present in Sector 1 in Fusion, as the Federation created it primarily as part of their Metroid breeding program and study of their natural lifecycle.
 * Despite the official guide stating that the purple liquid's purpose is to keep Metroids from reproducing, the Metroid Queen was nowhere near the last known location of the harmful substance. As previously mentioned, there is also no presence of said liquid, nor any evidence that it may have once been present between her nursery and the surface of SR388. Therefore, the Queen would have been well capable of producing Metroid Eggs without hindrance.
 * In Metroid II: Return of Samus, if Samus has collected sufficient Energy Tanks, she can dive into the hazardous substance for an extended period of time and reach other areas of the game in a non-sequential order. Reaching these earlier than intended reveals that the substance usually halts near the area's entrances, allowing Samus to emerge from the liquid, explore the locations safely and recharge her health with pickups dropped from enemies she finds and slays. This can be considered a sequence break as it allows the player to skip entire areas.
 * The final area, where the Queen Metroid resides, is the only one in the game of which's entrance is entirely sealed off by a spiked structure, and Samus does not have enough time to make it back to the previous Phase 8 before the harmful substance kills her, even if she carries the maximum amount of Energy Tanks. The spiked structure disappears following the earthquake triggered by the death of the final Omega Metroid.
 * Although Metroids do not live in rooms where purple liquid is present, the baby can still safely move through these rooms without touching it. This may indicate that the creatures simply prefer to build their nests in less toxic areas.