Microbombs are small explosives placed on a person's body and set to explode whenever that individual dies, meant only to destroy that specific person's body. They are used to make sure that neither a person's body nor important items can be retrieved by the enemy and were primarily utilized by the Cobra Unit.
Microbombs were activated by The Pain, The Fear, The End and The Fury when they were all killed during Operation Snake Eater. It was also suspected that they carried them around even though they weren't in hostile territory because they were ready to die, due to no longer having a unit to belong to or even a country to call home, and could only find it fitting to die on a battlefield.
Behind the scenes
Microbombs are detonated when The Pain, The Fear, The End, and The Fury, are killed during the events of Metal Gear Solid 3: Snake Eater. Although The Fury's death is depicted in a surreal manner, his microbomb can be seen to detonate after his fiery remains pursue Snake into the passageway leading from Groznyj Grad's underground tunnel. The nature of the microbombs is revealed if the player contacts Major Zero by radio after defeating them. Snake's question will also vary slightly depending on when he calls them. If he had only fought one Cobra, Snake will ask "Why did he explode?", if the call was made after having fought at least two Cobras, Snake will begin the conversation by asking, "Why do they keep exploding!?"
Although it is stated that the microbomb activates when the individual dies, The End's does not activate immediately should the player wait seven days for him to die of old age. Instead, it detonates shortly after Snake discovers his body and walks away, with The End's parrot also escaping the blast moments beforehand.
While the microbomb is designed to destroy the body and personal effects of the Cobra using it, for gameplay reasons a stamina kill still results in that member dropping their camouflage, or in the case of The End, his Mosin-Nagant tranquilizer rifle.
In The Joy, a non-canon Secret Theater film from Metal Gear Solid 3: Subsistence, The Boss activates a personal microbomb in mid-air after being attacked by The Pain's hornets and falling from the bridge in Dolinovodno. She explodes in a manner similar to the rest of the Cobras, exclaiming her former codename of "The Joy" as she does so, much to Colonel Volgin's confusion. In the clip Diehard, Volgin himself explodes after he is launched into the air by Ocelot's motorcycle, suggesting that he may have also possessed a microbomb in this scene.
Microbombs appear again in Metal Gear 2: Solid Snake, and are seen used by several of Zanzibar Land's mercenaries, including the Black Ninja, Running Man, and Night Fright. In each case, the mercenaries use microbombs to destroy themselves and their equipment, though several of them do not use them.
Although microbombs themselves don't reappear in any other Metal Gear games, a similar function is seen with various cyborg enemies in Metal Gear Rising: Revengeance. A Codec call to Doktor explains that the cyborgs exploding are the result of a self-destruct protocol that would activate if they fell in battle, in order to prevent their slayer from gaining any important data from them. Unlike the microbombs, however, the self-destruct sequence does not activate if they end up taken by surprise and ambushed. While defeated bosses exploding upon death were previously featured Metal Gear 2: Solid Snake, there is no explanation for this outside of gameplay purposes, although the survival of Big Boss and Gray Fox in subsequent sequels suggests that they did not actually die in such a manner.