Battlefield Methane Tanks
From Halopedia, the Halo wiki
File:MethaneTank.jpg
A Batttlefield tank
The Grunt's homeworld is a methane planet and as such they breathe a methane atmosphere. Thus when they are on an oxygen atmosphere like the Halos or Earth they need to wear a methane gas mixture. When Grunts are in battle, they will run low on their supply of methane, so they must have a ready supply nearby.
They are short and round in shape, purple in color and have a large blue valve on top. They are seen generally where Grunts are operating. They only appear in Halo 2[1].
The tanks are highly explosive due to their content.
