[A/N: Not going to lie, the wiki explains this much better than I would have, so I shamelessly copied and pasted quite a bit of the lore underneath]

Fallout, is a world much like our own that takes a drastic turn right around the end of world war two.

Although events in the Fallout universe and the real world diverge around the late-1940s, the defining moment of the Fallout universe was the Great War of 2077. Dwindling petroleum reserves across the planet led to a series of regional wars for resource control, and economies began to collapse as most nations were left without this vital lifeline. Alternative sources of energy were explored, leading to the maturation of nuclear fission technology, as well as various innovations in solar and hydroelectric power, and even nuclear fusion, but none of these options were sufficient to serve the global population's ever-growing need for energy.

With the available reserves of crude oil in the world diminishing, the communist government of the People's Republic of China declared war on the United States of America, invading Alaska in the hope of capturing the few remaining sources of crude oil there. The Sino-American War, as it came to be known, raged for eleven years, eventually culminating in a full nuclear exchange between China and the United States. Both nations had built up huge stockpiles of nuclear weapons over the years, and the mutual attack drew in enemies and allies from all across the globe, igniting a powder keg that had been building throughout the century-long cold war. Although the conflict lasted only two hours, the destruction it brought was staggering and utterly complete. More energy was released in the early moments of this exchange than in every previous global conflict combined. This global nuclear conflict came to be known as the "Great War".

What little remained when the smoke cleared was harsh and unforgiving. Ninety years after the Great War, humanity struggled to survive. On the surface, bandits and organized gangs fought over the crumbling remains of once-great cities, and mutants prowled the irradiated wastes, taking refuge in places too tainted for human habitation. Underground, a "fortunate" few enjoyed the relative safety and comfort of the Vaults, designed before the war with the stated purpose of protecting residents and their descendants from nuclear annihilation by the Pre-War company, Vault-Tec. Some planned to keep the world out, others sought to connect with and repopulate the outside. Wherever humanity survived, it was under constant threat by ravenous mutants, rogue machines, vicious raiders, and all manner of hostile mutant creatures.

There were a great number of known and unknown Vaults, but in the Majove Wasteland, just north of the ruins of Vegas lay Vault 22, an Experiment Vault.

Not all vaults were created to be bastions of humanity, quite a few were houses of horrendous experiments that showed the worst and very rarely, best of humanity. These experiments ranged from testing advanced Cryo technology that would document how humans would be affected by prolonged cryosleep to experiments that would have a Vault pumped filled with hallucinogenic drugs that would drive everyone in the vault mad and lead the residents of the vault to slaughter each other and everyone who entered.

Before the War, Vault 22 contained scientists working under the auspice of experimentation with staple crops to combat global hunger. The entrance sign describes the projects underway within, from developing advanced fertilizers to improving crop production and increasing these crops' resistance to insects, drought, and disease.

After the War, the scientists within the Vault continued agricultural-based initiatives, with the goal of sustaining the Vault population with plants grown within its confines.

Designed as a green Vault, people selected for populating the Vault were dedicated to such goals, and some of their most successful experiments were donated by defense contractors, such as the fungus that would eventually become the Vault's undoing.

Beauveria Mordicana was originally an entomopathogenic fungus designed for pest control. When a pest is exposed to the fungal spores, the fungus infests it and begins to colonize the host body. The host eventually dies due to failure of body functions, as the colony expands through its body.

However, the dead body continues to be controlled by the fungal colony, allowing it to move around and infect more pests by spraying spores around the host body. While effective in the long term, it still takes between 10 to 20 days to kill its prey and achieves limited effectiveness when dealing with unsocial pests.

The fungus was originally developed at the X-22 botanical garden at Big MT, though the technology was eventually shared with Vault-Tec, to enable botanical experiments at Vault 22.

As with many technologies coming out of Big MT, the fungus was a Pandora's box. Once the Vault scientists began experimentation with the fungus after the Great War, they effectively signed their death warrant.

While ostensibly designed to provide effective means of long-term pest control, the fungus was perfectly capable of infecting human bodies. Fungal spores gradually spread through the Vault, slowly infecting the population.

The first confirmed infection was Dr. Harrison Peters, who also provided insight into the development of the infection. It begins with pneumonia as the fungus invades the lungs, eventually transforming into chills, a fever, and a terrible racking cough. The fungal colony causes the body to actively reject anti-fungal treatments, leading to death due to organ failure.

As with smaller pests, the fungal colony continues to grow and develop. Eventually, it spreads far enough to establish control over the deceased person's body, becoming a highly aggressive spore carrier, a mindless beast whose sole purpose is spreading the spores. The untreatable infection baffled the scientists and spread through the Vault like wildfire.

The widespread infection led to a collapse of social order in the Vault. Infected humans were turned into spore carriers, with the airborne fungal spores threatening the remaining living residents.

Scientists attempted to eradicate them by igniting gas deliberately released in the lower levels of the Vault. However, the release triggered an incredibly aggressive reaction from the carriers, which attacked the scientists, tearing them limb from limb.

By 2096, the Vault was finally abandoned and a party of 118 survivors made way for the Zion Canyon. The dead eventually mutated into spore carriers or became stationary colonies of the fungus.

The dwellers traveled to Zion Canyon. With over one hundred men and women, strong discipline, tactics, and firepower, the small band of Mexican survivors camped out there posed no threat to the dwellers. They hit the survivors' camp on February 11, 2096, killing all the men and shooting down women and children that resisted. The remainder was taken to the main camp of the dwellers and penned in like livestock.

Randall Clark, the guardian of the valley, was desperate to help the Mexican survivors. Over the next two days, he surveyed the area. Vault 22 dwellers were organized well, with patrols and sentries set up along all approaches into the camp, except for the stream. The coughing among the dwellers puzzled Clark, but he ignored it as he prepared to save the surviving Mexicans. He went into the camp on February 14.

What he saw snapped something inside him. The Vault 22 dwellers killed and ate everyone they took from the Mexican survivors' camp. Clark retaliated, waging a brutal war of attrition against them. With his rifle in hand and all the explosives he could use, he ambushed the dwellers where he could and booby-trapped bodies and weapons he could not take.

By the end of February, he had killed 24 dwellers. In the ten days it took to accomplish this, he only suffered one wound: a 10mm steel jacket round through the thigh, missing the femoral artery. Though he remained unseen by survivors, he was forced to move camp when on March 2, a small patrol nearly uncovered his camp in the cave when one of their men was caught in a deadfall trap.

Panic fire almost hit him. After he disposed of the intruders, he moved camp to Cueva Guarache. Victory came ten months later. After losing six concurrent overseers and more than eighty members of the group to Clark and sickness, the dwellers gave up and fled the canyon, after eating their dead for nourishment. History loses track of them past that point. Occasionally, damaged Pip-Boys and Vault jumpsuits from the Vault would find their way into the hands of prospectors.

The Vault itself fell into disrepair over the years, overrun by the plants and wildlife that thrived in the conditions within. The occasional scavenger quickly fell prey to the creatures dwelling within or the fungus, if they managed to survive. It wasn't until roughly 2281 (around the time when Fallout New Vegas's events take place) when it piqued the interest of others. More specifically, Thomas Hildern, director of the NCR's Office of Science and Industry, or OSI East.

He had hired multiple mercenaries to retrieve the secret of the Vault, hoping to find a way to enhance the agricultural output of the NCR and stave off projected famine. Of course, the biggest benefit would be a major promotion for him. His latest hire, Keely, has gone missing much like the previous mercenaries, but this time, the situation is different.

Big MT, or Big Mountain on the other hand, where the Beauveria Mordicana was created, was a safe haven for scientists and the birthplace of countless advanced and unbelievable technologies that ranged from teleportation to forcefield technology, advanced robotics, weapons, armor, and all manner of technology.

However, the scientists at Big MT proved too dangerous, having no morals that prevented them from harming, torturing, or experimenting on their fellow man if it meant they could advance in their technology, and the US government did nothing to stop them or prevent the tragedies they committed against their fellow man if it meant they could have better weapons to use against the Chinese.

Doctor Mobius was one of the Big MT "Think Tank" executives before the Great War. Sometime after the War, he became a think tank in order to continue his work at Big MT. Like the others, he forgot a part of his personality and his real name, due to the recursion loop in their new perception programming.

After the escape of the cazadores and night stalkers, Mobius became disillusioned with the methods of the other think tanks. He grew tired of the endless and horrible experimentation and was deeply afraid of the damage the amoral and psychosis-ridden think tanks would wreak upon the fragile, post-apocalyptic world in the name of science. He erected a radar fence, a kind of radar-guided repulsive field, around the perimeter of Big MT to prevent the disembodied brains from escaping into the wasteland.

After a severe dispute with Klein, Mobius removed all the chips of all personalities of The Sink; his room and personal laboratory, and against their will, hacked the data banks of his colleagues, reprogramming their chronometers, geometers, and cartography programs, making them lose their sense of history, time and the world beyond Big MT to the point that they now believe Big MT is the world and nothing exists beyond its mountainous boundary.

He also actively fostered the false notions that robo-scorpions are somehow able to consume intelligence from brains, fueling the Think Tanks' fear of leaving Big MT's central dome. In the process of making these alterations, though, he also damaged some of their memories and knowledge, leaving only mangled, disjointed versions behind. Disgusted to have done this to his friends, he also erased some sections of his own memory, like this event and his pre-War creation, the robo-scorpion, before exiling himself.

Afterward, Mobius moved to the X-42 robo-warfare facility (which he renamed the "Forbidden Zone") and began to "recreate" the robo-scorpion, inspired by radscorpions that had wandered into Big MT.

These new versions included all features of the pre-War model, such as an energy bolt blaster in the tail, highly sensitive robotic eyes, and communication systems connected directly to his stronghold. Now in possession of a proper army, Mobius decided that without something to distract the other think tanks, the radar fence would eventually become insufficient to contain them.

He began to send repetitive, threatening broadcasts, designed to instill fear and prompt the think tanks to focus on retrieving technologies with which to defend themselves. With the remaining think tanks trapped in the central dome by their exaggerated fear of Mobius, the Big MT fell mostly silent.

Although not originally aggressive in his intentions, Mobius' own behavior seems to have degraded over the years, likely in part due to an increasingly serious addiction to Mentats and Psycho, drugs that affected the mind. By 2281, he had not only lost the functionality of his right "eye" monitor after trying to fix it with a hammer, but also began to suffer from short-term memory loss, hallucinations, and obsessive-compulsive behavior.

Although his plan had been mostly successful, the recent arrival of Father Elijah, Christine, and Ulysses from the Mojave has caused the curiosity of the think tanks to begin pushing beyond the boundaries he had set for them.

The arrival of the Courier and the subsequent survival of their lobotomy proved to be the last straw. Mobius unleashed a fresh wave of robo-scorpions and announced his false intention to finish the other think tanks off once and for all, while subliminally suggesting to them that various technologies around the Big MT would help them against him.

These technologies, a sonic emitter, an X-2 antenna, and a Stealth suit Mk II are actually all the components required to rehouse a brain inside a body, everything the Courier needed to get his brain back from the very scientists he was helping that originally removed his brain from his body. Eventually, the Courier defeats the X-42 giant robo-scorpion, enters the Forbidden Dome, and confronts Mobius, and depending on their choices, would either kill the Dr, kill the scientists of Big MT, or possibly even both.

In the northwest section of the Big MT, a long-defunct research station, X-22 focused on researching plant life and its military applications was housed in a canyon. It was here that Beauveria mordicana was created, and later transferred to Vault 22. The first subject of experiments with the plant, Patient Zero, still stalks the abandoned ruins of the facility, even after two centuries along with a few spore barriers, and spore plants. Due to time and the degradation of the memories and minds of the think tank scientists, all knowledge related to how the Beauveria Mordicana was created was lost, and all that is left of the research station, is rubble.