Yellow liquid

Yellow liquid is a thick liquid seen in Area 8 of SR388.

Properties
Yellow liquid first debuted in Metroid II: Return of Samus, where it appears colorless due to the original GameBoy's lack of color palette. It is non-corrosive, unlike purple liquid, so Samus Aran can wade through it without her Power Suit taking any damage. However, it is much thicker than water, which is seen in Phase 3 and Phase 7. Therefore, it weighs Samus down considerably, impairing her ability to jump and completely negating the use of her Spider Ball.

Yellow liquid returns in the official remake, Metroid: Samus Returns. Here, it is functionally identical to water and has no unique effects on Samus's movement. However, in this remake, water now acts as it did in later games such as Super Metroid: it slows down Samus's movement speed and hinders her jumping. These negative effects are negated once Samus acquires the Gravity Suit (which did not exist in the original game) in Area 5. Therefore, it is likely that skipping the Gravity Suit would cause yellow liquid to similarly impair Samus's movement, just like in the original game.

Locations
In Return of Samus, the yellow liquid first appears in Phase 1, collected in small puddles in the tunnel leading to the first encounter with an Alpha Metroid. Phase 1's close proximity to Phase 9 might suggest that both sources of liquid may possibly be derived from the same source. Hornoads live near the liquid, hopping in and out of it without a problem. No such liquid is seen on the Surface in the remake.

In Samus Returns, yellow liquid can be seen in the background of Area 6, in the room where Samus fights the first Omega Metroid. It is seen nowhere else in the area. This may suggest that yellow liquid, much like Metroid webs, may be an example of Metroids warping their environment to resemble their preferred habitat. The yellow liquid is dripping down from the ceiling; as an alternative explanation, since Area 6 is located below Area 8, it could simply be seeping down from Area 8.

In both games, the yellow liquid makes its most prominent appearance in the narrow tunnels winding upward to a massive open cavern in Area 8. Large pools of the liquid have collected in these tunnels. In Return of Samus, if Samus steps in certain pools, she will fall through fake ground, and the thick liquid will drag her down to the bottom of the tunnel network. The bottom of the large cavern is flooded with the liquid, and Samus must use the Space Jump to break out of it. In Samus Returns, the former feature is replaced with Pit Blocks, and the yellow liquid is only seen in the background of the large cavern as Samus makes her ascent.

Trivia

 * In Samus Returns, when Samus first encounters a Zeta Metroid in Area 4, it is seen drooling a similarly-colored yellow saliva. It is unknown if the two liquids are related. If so, it suggests that Metroids produce the yellow liquid.
 * In Samus Returns, there are pools of greenish liquid in the background of the Metroid nest in Area 8. It is unknown if the two liquids are related. If so, the green liquid may be filtered into a more yellow hue as it seeps from the nest down into the rest of Area 8.