Black Mesa Incident

The Black Mesa Incident is a key event, the "incident" depicts the invasion of several races of intelligent extra-terrestrials into a large research facility located in the New Mexico Desert. They are able to mount the invasion via an expanding dimensional breach caused by an experiment carried out by one of the facility's scientists.

Beginning
The Black Mesa Incident was caused directly by an experiment in Sector C Test Labs' C/33a Test Chamber. The experiment was supposed to analyze the purest sample of Xen crystals, procured by expedition teams in the borderworld.

The GG-3883 sample was, in addition to being the purest, also the most unstable Sector C staff experimented on. Seeing this as a rare opportunity to gain priceless data, they boosted the anti-mass spectrometer to 105% power, much higher than the equipment's safety buffer zone of below 90%. Dr. Gordon Freeman was the scientist responsible for inserting the sample for analysis. As Freeman was preparing to do so, one of the overseeing scientists noticed a small discrepancy in the readings, but dismissed it as "within acceptable parameters".

Upon receiving the sample, Freeman proceeded as ordered and inserted the sample. As the sample touched the machine's beam, a catastrophic resonance cascade began.

Resonance cascade aftermath
The cascade had unimaginable consequences. Aside from damage to the facility, swarms of aliens began to appear from thin air, teleported from the borderworld the crystals originated from. Thrust into an alien environment, the creatures were highly aggressive and attacked the Black Mesa personnel on sight, which resulted in extreme casualties among science and security personnel.

Among the mayhem, three survivors make their way to the surface and call in the military to help them with the alien onslaught. The survivors are Dr. Gina Cross, Dr. Colette Green and Dr. Rosenberg. The military, in the form of the Hazardous Environment Combat Unit arrives quickly, bringing in troops, ordnance and vehicles via aircraft and trains. However, apart from dispatching the aliens, they are also instructed to silence the facility personnel, a task several are reluctant to do, but follow the orders nonetheless.

The HECU's foray into the facility is successful, as they manage to establish total control of Black Mesa's surface facilities as well as much of its underground, where they set up several fortified outposts to keep the aliens at bay. They also manage to capture Dr Rosenberg, who sent the distress signal, and lock him in a cargo car for interrogation.

While HECU establishes their presence in the underground, Dr. Green and Dr. Cross work to prepare the satellite delivery rocket for launch, with the aim of reducing the effects of the resonance cascade. However, the launch is aborted when soldiers capture the control room and massacre its personnel.

Barney Calhoun, a low-ranking security guard, manages to survive the initial events. Determined to escape the crumbling facility, he locates Dr. Rosenberg with knowledge gained from his associate, who was mortally wounded by HECU troops. With his aid, Rosenberg locates the old, abandoned A-17 Prototype Test Labs used for teleportation experiments. Calhoun helps the doctor by acquiring several key components of the machine, and narrowly escapes with him and his two colleagues, Walther and Simmons, just as the HECU storms the underground facility.

Escalation
The battle between HECU troopers and Xen invasion force escalates. Despite having control of the facility, the soldiers find the aliens to be an even match to them, with heavy losses sustained on both sides of the conflict. Heavy ordnance is being utilized, in the form of upgunned M2 Bradleys, M1 Abrams main battle tanks, mortars, F16s, airstrikes and artillery bombardment. The facility slowly becomes a true warzone.

As the battle rages, Freeman slowly becomes a top-priority target for the HECU, as he constantly manages to avoid capture and survive encounters with the military, part of which was the launching of the satellite delivery rocket prepared by Dr. Gina Cross and Dr. Collette Green. Shortly after, following Freeman surviving the flooded and highly damaged old hydrofauna research laboratories, he is captured by HECU troopers. However, instead of being taken for interrogation, he is dumped by two marines in a trash compactor. He narrowly escapes death and manages to work his way through residue processing areas and eventually reaching the Lambda Complex.

While Freeman progresses through the surface facilities, the conflict begins to favor the Xenians, as heavy Alien Grunts and Xen aircraft (the powerful manta rays) begin to arrive, shooting down one of the reinforcement fleets carrying Adrian Shephard's squad. HECU soldiers find themselves slowly overwhelmed, and the battles become increasingly bloody, with the troopers acting more erratic by the minute. Finally, a decision is made by the command to evacuate the facility and commence airstrikes. The evacuation is not completed, however, leaving many soldiers and equipment behind. With the failure of the military force, the black operations contingent arrives, to clean up the mess and destroy the overrun facility with a high-yield nuclear warhead.

Shortly after the evacuation begun and black ops arrived in full force, Freeman gained access to the Lambda Core complex and teleported to the Borderworld, eventually killing the Xen leader, the Nihilanth.

Last hours
As the black operations operatives assessed the situation (for example, by interrogating stranded HECU troopers), an unknown foe arrived, the Race X. Meanwhile, one of surviving HECU members, Corporal Adrian Shephard, manages to fight through hordes of Race X aliens, trying to find an escape from the infested base, witnessing first hand the results of multiple alien/military engagements. Eventually, he faces the Black Ops and is able to defeat much of them, much like Gordon Freeman did when facing HECU soldiers.

Wandering through the facility, he helps several members of the science personnel and fellow marines, facing off against several squads of the Black Ops intent on destroying the facility with a Mark IV Thermonuclear Device, capable of destroying everything in over a 50 km radius, effectively obliterating the base, remaining soldiers, science team members, security guards, Xenians and Race-X infestation.

The device succeeds in destroying the base, after being re-armed by the mysterious G-Man following Cpl. Shephard disarming it.

Aftermath
The results of the Black Mesa Incident were massive. The relatively small inter-dimensional rift caused portal storms all over the Earth, bringing large amounts of Xen wildlife to the planet and, most importantly, attracting the Combine Empire, which attacked and enslaved Earth, following a brief, seven hour war.

To date, there are at least ten survivors of the incident, among them Dr. Eli Vance and his daughter Alyx, Dr. Wallace Breen, Dr. Isaac Kleiner, Dr. Gordon Freeman, Dr. Rosenberg, his colleagues Walter and Simmons, security guard Barney Calhoun, and HECU Corporal Adrian Shephard. Whether they all escaped the facility before the bomb was detonated is unknown. Whether they all survived the Combine Invasion of Earth is also unknown.

It is unknown whether Dr. Richard Keller, Dr. Colette Green, and Dr. Gina Cross survived the incident as Decay doesn't reveal their fates. It is strongly possible, if they didn't already die or leave, that they are killed by the nuclear device at the end of Opposing Force.