Lava

Lava (or Magma) in the world of Metroid is a hazardous and viscous substance of molten rock that harms or kills things that come in contact with it. It appears in all games in the series except Metroid Prime 2: Echoes and Metroid Prime 3: Corruption (though Dark Water in Echoes and Fuel Gel in Corruption seems to take lava's role of hazardous substance). Samus' speed when running through it is slowed down in Super Metroid, even with the Gravity Suit on. This is likely due to the viscosity. However, in Metroid Fusion and Metroid: Zero Mission this effect is non-existent unless Samus is not wearing the Gravity Suit. Some areas in the original NES Metroid and Zero Mission feature fake magma where Samus can fall right through to reach secret areas without being harmed. Once entering the fake magma, it will vanish revealing it to be no more than an illusion or possibly holograms. The Diamont Bounty Hunter Spire is immune to the damaging effects of lava, allowing him to move freely through it without being harmed as if he were wearing the Gravity Suit.

When Samus acquires the Gravity Suit in Zero Mission, Super Metroid, and Metroid Fusion, she develops immunity to the magma or lava. However, some superheated magma in Ridley's Lair in the depths of Norfair can still harm her in Super Metroid for unexplained reasons. The same applies to Metroid Prime's and Metroid: Other M's lava within the Magmoor Caverns/Pyrosphere.

In Metroid II: Return of Samus, Acid exists on SR388. Sector 1 on the Biologic Space Laboratories research station, however, contains lava in areas mimicking the planet.

While the original Tourian featured in Zero Mission had normal lava that the Gravity Suit could protect Samus from, the rebuilt Pirate Headquarters in Super Metroid is filled with two different lava-like substances that ignore the mentioned suit's protection: the first one seems to be a highly concentrated liquid not found anywhere else on Zebes, while the second is identical to the superheated magma found mostly in Lower Norfair.

Trivia

 * Lava and Metroid were referenced in the film Spy Kids 3-D. In one scene, the characters were discussing the constant presence of lava in video games. One says that "In Metroid, it's actually magma."
 * The mentioned superheated substance found mostly in Ridley's Lair was capable of melting the entire body of the Crocomire, leaving nothing but its bones.
 * In Metroid: Other M, Samus is very limited in her actions while submerged in the substance; she cannot engage Morph Ball mode and is unable to jump out of lava depending on her position.
 * Sovas "drink" lava.
 * According to concept art for the Sovas in Metroid: Other M, there are apparently microbes that are not only able to resist the extreme temperatures of lava, but thrive in it as well. As Sovas are native to Zebes and feed on them, one can assume that the microbes are native to there as well.