Black Mesa incident

The fictional Black Mesa Incident is a key event appearing in the first-person shooter computer game Half-Life. The "incident" depicts the invasion of several races of intelligent extraterrestrials into a large research facility located in a New Mexico desert. They are able to mount the invasion via an expanding dimensional breach caused by an experiment carried out by the facility's scientists.

Beginning
The Black Mesa Incident was caused directly by an experiment in the Sector C 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 sample that Sector C staff experimented on. It is implied in Half-Life 2 Episode 2 that the G-Man presented this crystal to the scientists. 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%. The scientists admitted that it was "a bit of a gamble," but they needed extra resolution for the analysis. 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 "well within acceptable bounds again".

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.

The Resonance Cascade
The cascade had severe consequences. Aside from damage to the facility, swarms of aliens began to appear from thin air, teleported from the borderworld that 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.

During the mayhem, three survivors made their way to the surface and called in the military to help them with the alien onslaught. The survivors were Dr. Gina Cross, Dr. Colette Green and Dr. Rosenberg. The military, in the form of the Hazardous Environment Combat Unit arrived soon thereafter, bringing in troops, ordinance and vehicles via aircraft and trains. However, apart from dispatching the aliens, they were also instructed to silence the facility personnel. Several of them were reluctant to do, but followed their orders nonetheless. The journal entries of Adrian Shephard found in Half-Life: Opposing Force's instruction manual imply that the HECU was preparing for the Black Mesa Incident well in advance of the Resonance Cascade.

The HECU's foray into the facility was successful, as they managed 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 managed to capture Dr Rosenberg, who sent the distress signal, and locked him in a cargo car for interrogation.

While HECU established its presence in the underground, Dr. Green and Dr. Cross worked to prepare a satellite delivery rocket for launch, with the aim of reducing the effects of the resonance cascade. However, the launch was aborted when soldiers captured the control room and massacred its personnel.

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

Escalation
The battle between HECU troops and Xen invasion force escalated. Despite having control of the facility, the soldiers found the aliens to be an even match. Heavy losses were sustained on both sides of the conflict. The HECU weapons included upgunned M2 Bradleys, M1 Abrams main battle tanks, mortars, F16s, airstrikes and artillery bombardment. The facility became a true warzone.

As the battle raged, Dr. Freeman became the top-priority target for the HECU, as he managed to avoid capture and survive multiple encounters with HECU fire squads and armor, as well as launch the satellite delivery rocket prepared by Dr. Gina Cross and Dr. Colette Green. Freeman then survived a journey through the flooded and highly damaged old hydrofauna research laboratories, but was then captured by HECU troops. However, instead of being taken for interrogation, he was dumped by two marines in a trash compactor. He narrowly escaped death and managed to work his way through residue processing areas and reached the Lambda Complex where he teleported to the Borderworld and eventually killed the Xen leader, the Nihilanth.

While Freeman progressed through the surface facility, the conflict began to favor the Xen forces, as heavy Alien Grunts and Xen aircraft (manta-ray-like objects that function as powerful combat platforms as well as transport for some of the larger Xen grunts) arrived, shooting down one of the reinforcement fleets carrying Adrian Shephard's squad. HECU soldiers found themselves overwhelmed, and the battles became increasingly bloody, with the troopers acting more erratically by the minute. Finally, a decision was made by the command to evacuate the facility and commence airstrikes. The evacuation was not completed, however, and left many soldiers and much equipment behind. With the failure of the military force, the black operations contingent arrived to clean up the mess and destroy the overrun facility with a high-yield nuclear warhead.

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 the surviving HECU members that were left behind, Corporal Adrian Shephard, managed to fight through hordes of Race X aliens, trying to find an escape from the infested base, where he witnessed first hand the results of multiple alien/military engagements. Eventually, he faced the Black Ops and defeated many of them.

Wandering through the facility, Adrian helped several members of the science personnel and fellow marines, while facing off against several squads of the Black Ops intent on destroying the facility with a Mark IV Thermonuclear Device.

Cpl. Shephard disarmed the bomb, but it destroyed the facility anyway, due to the G-Man re-activating 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 the Seven Hour War.

At least eleven survivors of the incident are known, 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, Dr. Arne Magnusson, and HECU Corporal Adrian Shephard. To date, the only ones not to have been seen since the Combine takeover are Doctor Rosenberg, Walter, Simmons, and Corporal Shephard, who was detained by the G-Man at the end of Opposing Force.

It is unknown whether Richard Keller, Colette Green, and Gina Cross survived the incident as Decay doesn't reveal their fates. It is entirely possible that they were killed by the nuclear device at the end of Opposing Force.