Unggoy combat harness

The Unggoy Combat Harness is a type of body armor issued to the Covenant's Grunt warriors. It provides basic protection from hostile fire and the elements, and is also equipped with a breathing apparatus for the methane breathing aliens like Grunts.

Breathing Apparatus
The gas is stored in super cold tanks inside the suit, and is provided to the user through a breathing mask that covers over half of the alien's head. The triangular backpack attached to a Grunt's back contains a small Methane tank. Attached to every Grunt's wrist is a readout (measured in Covenant units) of the amount of methane left in the Grunt's pack. When the tank gets low, a resupply of methane must be found quickly lest the Grunt will suffocate. The suit also has the color of the Grunt's rank on display. In Halo 3, both the breathing mask and backpack component of the suit can be knocked off while the Grunt is still alive or dead; this causes the Grunt to suffocate, if still alive. It is interesting to note that in Halo 3, even if Grunts' lose their Methane tank on their back, they can continue fighting without suffocating. This may mean that there is emergency Methane, stored inside their Combat Harness. However, this is more likely just for game play reasons. The mask cannot be knocked off in any other game.

Defense
In addition to its life support role, it is equipped with armor. This armor can withstand limited amounts of fire, especially from energy weapons. The M6D is capable of easily cracking through the armor and is considered by some to be the standard weapon for such actions. The armor piercing rounds of the Assault Rifle are also potent weapons against the harness. As well as defending against projectiles and plasma, it prevents a limited amount of fall damage to its user, like its surpassing counterpart. Despite this, most variants are vastly inferior to the other types of armor adorned and used by the Covenant. Some variants are also equipped with helmets, Active Camouflage and a methane-filled breathing apparatus. A grenade can easily rip through the armor and kill the Grunt.

Trivia

 * Even though a grenade can tear off the breathing mask in Halo 3, you can't see the mask come off but you can see methane rise in the air.
 * There was a variant with a partially curled-back methane tank, along with a bigger face mask, but it is only seen in Halo: Combat Evolved surprisingly called by fans, "Curl-Backs".
 * In Halo 3, the breathing mask can be removed by projectiles and explosions.
 * You can also punch the methane tank off the back of the grunt without killing it in Halo 3.