Methane Tank

Methane Tanks are cylinder shaped structures used by the Covenant. They are large and resemble fuel tanks of modern trucks.

Methane Tanks are used for the Grunts of the Covenant. They provide a place for the creatures to breathe their methane atmosphere outside of their suits. Thus, Grunts usually remove their suits when entering a methane tank.

If the suit is removed while the grunt is in a low-methane environment, the grunt will gasp for air (like a fish) before dying.

There is a main chamber for the Grunts to reside in and a smaller chamber in front where the Grunts can change into and out of their suits. These structures often have posted guards to prevent the creatures inside from being caught off guard.

Halo: The Flood mentions these specifically, but in Halo: First Strike, edifices called "Methane Pits" are noticed by Spartan-104. They may or may not be the same, but that is unclear.

Trivia

 * In some Halo 3 screen shots, Grunts are seen without their methane masks.this could mean that they are able to be knocked off.