Total darkness

Total darkness is a recurring obstacle in the Metroid series.

Description
Samus Aran's helmet is equipped with an Infrared Ray Scope that allows her to see in the darkness, which is a necessary tool for exploring the dark caves and tunnels of various planets. However, there are some areas that are too dark for even her Infrared Ray Scope, forcing her to rely on other methods to navigate her way through total darkness.

Metroid Prime
In Metroid Prime, Samus acquires the Thermal Visor in the Glacier One Research Core. However, upon doing so, many rooms of Glacier One will lose power and become enveloped in total darkness, with only some glowing technology still visible. This also allows Tallon Metroids to escape their containment and increases the number of Shadow Pirate guarding the facility. Fortunately, Samus can use her newly-equipped Thermal Visor to navigate these dark rooms.

Whenever Chozo Ghosts spawn, the room that they occupy becomes considerably darker. While this adds to the atmosphere and makes it more difficult to navigate the room, it also makes it easier to see the brightly-glowing Chozo Ghosts when they are visible.

Some rooms in the Phazon Mines, such as Fungal Hall B, are also very dark with a little illumination provided by the Saturnines.

Metroid II: Return of Samus
Total darkness was first introduced as a gameplay element in Metroid II: Return of Samus. The Dark Chamber in Phase 7 received its name from the fact that it is almost entirely pitch-black. The only things visible in this chamber are Samus, her weapons, a Missile Pod, and a bit of light coming in through the two openings in the ceiling. Because her weapons are visible, Samus can most easily navigate the Dark Chamber by leaving a trail of bombs.

Total darkness does not return in the remake, Metroid: Samus Returns, with the redesigned Dark Chamber now being fully-illuminated.

Super Metroid
In Super Metroid, Samus can find caves inhabited by Firefleas in Brinstar and Ridley's Lair. At first, these rooms are well-illuminated by the glowing Firefleas. However, for every Fireflea that Samus kills, these rooms grow darker and darker, making them more difficult to navigate. If all the Firefleas are dead, then the room is fully enveloped in total darkness, with only Samus and a few environmental objects still able to be seen. Therefore, to avoid total darkness, Samus must navigate these rooms without killing the Firefleas.

Trivia

 * A downplayed form of total darkness can be seen in Sector 6 (NOC). The nocturnal environment is darker than any other sector of the Biologic Space Laboratories research station, with a small circle of light surrounding Samus. However, the rooms are still well-lit enough to easily navigate, and this lighting effect is hardly noticeable when the game is played on a backlit screen (i.e. Nintendo DS or Virtual Console).