Battlefield Methane Tanks

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 colour and have a large blue valve on top. They are seen generally where Grunts are operating. They only appear in Halo 2.

(Sorry, I couldn't find a picture of one. They're all over the place though)