Acid

Acid is a substance found throughout the Metroid series. Acid does harm to Samus' suit, and at a rapid pace. It also slows her movement. It is mainly found on Zebes. Once Samus has acquired the Varia Suit, she becomes immune to the effects of Acid. Certain creatures can live in Acid and not be harmed in the slightest. Ki-Hunters can spit this substance at Samus to damage her.