Gas container

A container can be seen behind two Heretic Grunts.

The Forerunner's mining facilities use Gas Containers to move the volatile gases from location to location within the facility. They can detonate if exposed to weapons fire or an explosion in their immediate vicinity. You can use these to kill Heretic forces on the level The Arbiter. They also spawn in the Halo 2 on the Multiplayer level Colossus. The gas inside these containers is possibly methane; it appears to be similar in color to that of which the Unggoy breathe.

The substance could also be a possible fuel used to power the station and the containment shields of the Flood perimeter.