The Lightmass Bomb is a bomb created supposedly from a process similar or related to Dr. Helen Cooper's Lightmass Process. While it is always referred to as a bomb, it functions as a cluster of highly explosive guided missiles/torpedoes in practice. Due to the nature of the Hollow and its Imulsion-flooded tunnels, this torpedo-based design was the most efficient way to deploy the Lightmass warheads. Consequently, the weapon requires an immense amount of data on the area between the launch point and targets to guide the individual projectiles. This was originally to be provided by the Resonator sonic mapping device, but when the device did not perform as planned, data collected by Adam Fenix had to be substituted. It is a very destructive weapon when used effectively as seen in the final cutscene after defeating General RAAM in Act 5: Desperation. After being deployed at Timgad Valley from the derailed Tyro Pillar, the projectiles successfully filtered through the underground tunnels and destroyed a significant populous of the Locust, as well as the Kryll breeding grounds.
As part of the plan to sink Jacinto in order to destroy the Locust Horde by flooding the Hollows with seawater, a second Lightmass Bomb was prepared as it was believed to be powerful enough to do the job. Cole and Baird remained behind to help prepare the bomb while Marcus Fenix and Dom Santiago entered the Hollows through a giant Locust sinkhole in order to open a hole to drop the bomb. They succeed, but their Brumak went Lambent and knocked the helicopter carrying the Lightmass Bomb into the Imulsion. Another plan had to be formed with the loss of the Lightmass Bomb, and ironically the Lambent Brumak was used as the bomb itself by detonating it with the Hammer of Dawn, completing the mission and apparently killing off the Locust Horde.
It is unknown what process allows the bomb to become so powerful, but it is surmised that refined (or possibly even raw) Imulsion has explosive properties. This would also explain why the Lightmass Process can harness energy from Imulsion, if it were explosive the energy released from said explosions would be usable, in a similar way to petroleum and diesel fuel, harnessing energy for the masses. It is very possible that torque bow arrows use a miniature Lightmass process, releasing the Imulsion's energy in an explosion.
Another hypothesis, which is somewhat substantiated, is that it uses a process related to the Lightmass Process to "detonate" Imulsion, giving it increased destructive power depending on how much Imulsion is present when detonated. This would mean that the Lightmass Bomb would be less of a bomb itself and more of a catalyst to the reaction created when it makes contact with Imulsion. This is quite possible, since Imulsion exposed Dark Wretches gain a similar, explosive quality.
- It has been revealed that the detonation of the Lightmass Bomb at the end of Gears of War has sent some Imulsion up into the atmosphere of Sera causing a condition known as Rust Lung to spread through the remaining Human population.
- The Timgad Valley, where the Lightmass Bomb was deployed, was home to the breeding grounds of the Kryll. The detonation of the Lightmass bomb destroyed the breeding grounds and killed virtually all of them, to the point where they are either extinct, or their numbers are too low to breed enough to ensure their survival as a species.