First the grand theory. Man is all powerful he builds cities controls the world and imulsion. Man fights himself primarily over imulsion. Man however gets sick if he gets too close to imulsion (Or irradiated whatever you want to call it). Man creates race of super imulsion miners that are more resistant, AKA the locust. Locust build cities underground and happily mine imulsion. Locust keep digging further down. Locust that have gone the furthest down become warped (AKA Lambent) maybe they discover some other power deep in the center of the earth. Lambent locust turn on the normal locust and say "*** you guys we are taking over". Normal locust say "*** you humanity we aint dieing down here for you, we're coming up to the surface to take over and drown the lambent". Lucust come up during E-Day kick humanities @@@.
Now to support my theory. First things first the games have made many hints that it's all about imulsion, this was hinted in gears 1 then it took a huge step forward in gears 2 when the collectable messages emphasized how important imulsion was, how the prices were raised during the war and one even hinted of a conspiracy theory for it (Kind of like our own world and oil). Also it was also pretty clear that the locust were man made, again this was reinforced with the lab level that was mysteriously "Declassified" and it's also pretty clear that Marcus's father was probably on the project as quoted by the locust queen saying "It's a shame you didn't follow in your fathers footsteps" and at the end of the credits he's alive and says "What have you done!" after you destroy the normal locust cities.
The last bit comes from a leaked script for the gears of war movie. It existed on the internet for a brief time about a year and a half ago and really explained a lot about what happened just before and during E-Day (It was a really good script and some scenes will be incredibly powerful if done right). Marcus's father was in the movie script and just before E-Day he was getting strange activities from the underground areas. One of the scenes had him trying to go underground to get some more data and talk to key people but the security guard was giving him a tough time saying "No one is allowed underground! Ever" which seemed kind of odd but why wouldn't you want people to see what was going on underground? If you created a race of supper miner slaves! That's why! Eventually the military called for Marcus's fathers help and started showing him videos of strange things that were happening underground because he was an expert in that area. Most likely because he was on the project and it would also make sense as to why he had all the locust tunnels mapped out in his home computers as per gears of war 1. Also it is my thought that the Queen Myrah was also a part of this project as in Gears 2 one of the collectables states a women named Ruth (Myrah's real name) was being experemented on in the classified facility and i beleive that she escaped to the locust hollow and then somehow became Queen and for what the humans did to her gained control over the locusts letting them beleive she wanted to help them when really all she wanted was revenge upon humanity which is quite obvious as you can see the attitude she displays to all humans.
Gears of War 1 beigns...
The game’s plot begins fourteen years after Emergence Day. Marcus Fenix, a former COG soldier, is reinstated into the military after spending four years in prison for abandoning his military post in order to make a vain attempt to save his father, Adam Fenix. Dominic Santiago successfully extracts Fenix from the prison, and takes him to meet Delta Squad. The group seeks to obtain the "resonator", a device that will map "The Hollow", the underground caverns which the Locust inhabit and later deploy the "Lightmass Bomb", which will destroy the heart of the Locust forces. Fenix and his allies recover the device, but suffer multiple casualties in the process. Amidst the chaos, Fenix leads the remaining soldiers through the ruins of Sera, to a mining facility, and finally into the planet’s depths. Delta Squad successfully denotes the resonator, but the device fails to map entire tunnel network. Fortunately, they discover a larger map of the network that originates from Fenix’s old home, specifically his father’s laboratory. The group ventures to the Fenix estate at East Barricade Academy, where Fenix originally attempted to rescue his father. When they arrive, Delta encounters heavy Locust resistance. After collecting the data, the group fights their way past Locust forces and boards a train carrying the Lightmass Bomb. Fenix and Santiago battle their way through the train, and defeat General RAAM, before finally uploading the data. Fully activated, the Lightmass Bomb launches into the Hollow, and eradicates the Locust tunnel networks. In the game's final sequence, Hoffman delivers a victory speech as the tunnels collapse and explode, whereupon the voice of the Locust Queen promises that the Locust will keep on fighting, and will not stop.
Gears of War 2 begins...
Taking place six months after Lightmass bombing at the end of the first game, the Locust Horde has survived and has renewed their war on Humanity, and through unknown means begun to sink entire Human cities. With the COG army continuing to lose large numbers to Locust raids, the COG plan a counter-attack against the Locust by sending troops in Grindlifts deep below the surface to attack the home of the Locust, the Hollow. Sending a large **** force through the mountains to the settlement of Landown, the COG take heavy losses from Locust opposition but make it to their destination. As they prepare to use the lifts to enter the Hollow they are driven back by a counter attack led by the Locust armies' new leader Skorge. Undeterred, the human forces continue their plan. After their descent, Delta squad, consisting of Marcus Fenix, Dom Santiago and Benjamin Carmine, continues to search the Hollow and discovers that the Locust are using a giant Riftworm to sink the human occupied cities. as they watch it sink Liima City, they also realize that the Locust plan to have it eat away at the rock structures at the outskirts of Jacinto in hopes of sinking it into the ground.
Exploring the sink hole, Delta Squad is ambushed by Locust and rescued by Augustus Cole. As they search for Damon Baird the squad discovers that the Locust have begun taking prisoners and taking them deeper into the hollow, with unknown intentions, after a quick search they find and free Baird. Boarding the Locust prison barges, Delta Squad finds another imprisoned squad member, Tai Kaliso, who has suffered wounds inflicted by Locust torture. Though Marcus tries to arm Tai for a fight, Tai instead commits suicide, the physical and emotional trauma was too much for him to bear. As the squad tries to be extracted by a King Raven Gunship, they are swallowed by the Riftworm. Inside the squad quickly plans to escape, but a plan by Marcus sets the squad to destroy the worm's heart to kill it. Inside, Carmine is mauled and eaten alive by monstrous parasites, having been separated from the rest of Delta. After destroying the worm's heart, the squad is irritatedto discover that the worm is survived by not just one heart but three—they nonetheless locate and destroy the remaining two, effectively killing the worm.
Delta squad narrowly escapes being drowned in the creature's blood by cutting their way out to the surface only to be given orders by Hoffman to investigate a nearby abandoned COG outpost where they encounter experimented creatures of unknown origin and learn of a back door entrance to the Hollow in the nearby Kadar Mountains. The team fights its way to the caverns in a Centaur tank and discovers that some Stranded have taken shelter near the entrance. Marcus orders Cole and Baird to evacuate the Stranded while he and Dom search further. They find more prisoner cages and Dom convinces Marcus that Maria must be inside one of them. They manage to find her, but the Locust imprisonment has left her too traumatized and unable to communicate, so Dom reluctantly chooses to euthanize her. Marcus and Dominic then disguise themselves as Theron Guards and continue their infiltration of Nexus, the Locust capital, where, they encounter and free a captive Franklin Tsoko, and later use Jack (the squad's utility robot) to signal for COG forces from the surface, who quickly join them, including Cole and Baird.
Inside Delta squad discovers that the Locust are locked in a civil war between the normal Locust and the "lambent," who have been exposed to Imulsion, and seek to over throw the queen, explaining the Locust's desperation to get to the surface to escape the overwhelming Lambent. Inside the Locust palace they discover that the Locust have messages from Adam Fenix, Marcus' father, that suggests an alternate way of ending the war, in case they can't take out the queen; to sink Jacinto in order to let sea water flood the Hollow and wipe out the Locust. They learn from the Locust queen herself that she is pursuing the same plan not only to wipe out the humans, but to also destroy lambent. After a face to face meeting with the queen, and a face off against Skorge, the queen escapes on a Reaver. Marcus realizes that humans must sink Jacinto first, before the Locust mobilize and escape to the surface.