Gas canisters are Forerunner-built devices. They were most noticeably seen at the gas mine and were presumably used by the Forerunner for the containment and distribution of gas, but some empty canisters were also seen on Installation 05. The gas could then be harnessed as a source of power, or stored for later purposes.
Gas canisters in-game come in two variations: empty and filled. The empty variant is a clear box and proves to be exemplary cover, albeit the fact that the bulk of them move on conveyor belts. The latter is filled with gas from the gas giant Threshold and possess a swirling blue appearance to it. When a filled canister experiences a breach, the gas violently escapes from it, producing a potent explosion as it disperses into the air.
As the player approaches the hangar bay docked with a Seraph, one may see that a lone Sentinel is carrying a canister. This may be accomplished with built in clamps, or magnets. Overall, the gas canisters appear in two campaign levels, The Arbiter, The Oracle, and in addition to that, empty ones can be found in the Halo 2 multiplayer map, Colossus.