Taking place several decades in the future, though designed with a charmingly retro aesthetic, Fallout is a series of games each set in a post-apocalyptic world ravaged by a nuclear holocaust. The U.S. government foresaw this catastrophe and funded a project to build a series of fallout shelters called Vaults. However, instead of acting as a safe habitation for survivors, the Vaults became sites for bizarre and disturbing experiments on its inhabitants. There never was any intention to keep people safe. In the end, only 122 Vaults were constructed, so the vast majority of the human population was left out in the cold (read: intense heat), while those unlucky enough to secure a spot in one of the Vaults survived a little longer, often in nightmarish conditions.
Each Vault was assigned an Overseer, who acted as leader in some varying capacity. In some Vaults, the Overseer was a mediator and advisor, while in others he or she would essentially be a dictator; in a couple, the Overseer wasn't even human, and in one, they were elected. Almost all Overseers were aware of their Vault's true purpose, and the details of the experiment being performed on their constituents. The experiments ranged from mildly unethical to straight-up horrifying. By the time most of these Vaults were opened, they were nothing but tombs. However, scarier still are the Vaults where something inside was still alive.
15 Vault 112 - Tranquility Lane
Created by Dr. Stanislaus Braun, the director of the Societal Preservation Program, Vault 112 was home to 85 people all suspened in a virtual simulation of Braun's own design. He had prototyped various simulations in the past, and the one installed in Vault 112 was called Tranquility Lane.
While conceptually it was meant to allow Dwellers to live perfect virtual lives, Dr. Braun had a more sinister purpose in mind. With adminstrative privileges over the simulation, he took on the form of a litle girl and terrorized his residents. For fun, he'd virtually kill them, wipe their memory, resurrect them, and repeat. Everyone in the simulation was trapped inside by a failsafe, and while Braun had the ability to kill them permanently, he himself could not die. Truly killing his residents meant living alone in Tranquility Lane forever.
14 Vault 68 and 69 - Gender Inequality
These Vaults may a paradise for some, but a nightmare for others. Entrance to these Vaults were evidently by special selection, given their unique specifications. Vault 68 was home to 999 men and one woman. Vault 69 was home to 999 women and one man. These numbers are taken from the well-known sex position. While details of what transpired in these Vaults, as well as their location, is unknown, it does leave some disturbing things to the imagination. Naturally, the population of either vault could not be sustained for more than one or two generations, leaving the Dwellers to die out before they could repopulate.
A similar Vault housed twenty men, ten women, and a panther. How they fared is anyone's guess.
13 Vault 77 - Puppet Man
Featured in an official web comic by Penny Arcade, this infamous vault is home to only one man. He was given food, supplies, and a single crate of puppets as part of the Vault's experiment. Only after over a year of panic and depression did he finally open the crate of puppets.
Inside he found a dog, a grandmother, and a king. Slowly slipping into insanity, he gave them each a personality and put on his own show, treating his puppets as real people. One day, he hears the voice of another puppet in the crate, that of a Vault Boy. Having initially overlooked this one, he begins talking to this seemingly homicidal puppet as more disturbing events unfold. Check out the comic for the full story here.
12 Vault 108 - Gary's Vault
Vault 108 was originally designed for observing power and leadership conflicts. However, it was also the only Vault (besides Vault 8) equipped with clone technology for reproductive and medical purposes. One man, Gary, created fifty four clones of himself for research purposes. However, certain flaws in the process caused each clone to be more violent and deranged than the last. As the Vault became overpopulated, the scientists opted to kill clone subjects to make room for more. But before this could happen, the clones escaped their confinement and took to killing any Dweller that wasn't Gary, eventually wiping out the Vault until only several dozen clones of Gary remained. When the player enters this Vault, all Gary clones, except for the original Gary and a couple outliers, will attack on sight.
11 Vault 22 - Killer Plants
Rather than house the subjects of an insane social experiment, Vault 22 was home to scientists, all working on a method to promote plant growth and sustainability while underground. Three species were originally used for the study: a common mantis, a kind of Venus flytrap, and a fungus.
While the spores of the fungus were useful in the study, they eventually proved to be toxic to humans, rapidly infecting the Vault's population. After killing the host, the fungus takes over the body and uses it to spread more spores, a chain reaction that forced the survivors of this Vault to flee the often violent spore carriers. After the Vault was abandoned, the other plants flourished and mutated into a more deadly form. Upon arriving at the Vault, the player is greeted by a "stay out" sign, warning them of the danger ahead.
10 Vault 29 - The Children Are Our Future
Through "accidental" redirection or careful selection of adults with terminal diseases, Vault 29 became home to only children under the age of 16. Rather than a human Overseer, the Vault would be managed by an AI supercomputer called the Zax. This robotic caretaker was meant to raise and educate these children so that they may one day be able to venture out into the wasteland and rebuild society.
However, these plans were changed by Diana Stone. Once a brilliant scientist, she was diagnosed with terminal cancer before the war. She underwent an operation in which her brain was connected to a computer. She found the experiment unethical, and at the start of the Great War, assumed control of ZAX using her new form. She sent robotic helpers out into the wasteland to build and prepare a village called Twin Mothers. When the children had matured, she sent them there to settle into their new life.
9 Vault 75 - The Children Are Our Future (Pt. 2)
Another sanctuary meant only for children, Vault 75 was located beneath a middle school. Any parents or teachers that made their way into the Vault were separated from the children and executed by security. A terrifying experiment then ensued, in which the children were tested to their physical and mental limits. Those who were too weak did not survive, and even those deemed unfit at "graduation" were also killed. The strongest children were "harvested" for their organs, for the purpose of creating superior genes. Those who showed intellectual promise were recruited as scientists to help with the experiment.
Eventually, one of the recently-graduated scientists helped stage a rebellion, leaving all residents of the Vault either dead or out roaming the wasteland.
8 Vault 87 - Forced Evolution
This Vault was the site of medical experiments in which scientists attempted to force test subjects to undergo biological changes that would allow them to survive in a nuclear wasteland. Subjects were locked in air-tight chambers and exposed to the Forced Evolutionary Virus (FEV). Many did not survive the experiment. Those who did increased in size and strength, while developing resistances to environmental dangers. This was, however, at the cost of their mental well-being; most turned violent and aggressive.
A year after the experiment's inception, a nuclear event led to the mutants escaping their confinement. Scientists who weren't killed were forcibly exposed to the virus. Mutants were deeply invested in the preservation of their kind, and because they were sterile, they took to kidnapping humans all across the Capital Wasteland and bringing them back to the Vault to be exposed to the FEV.
7 Vault 106 - Psychoactive Drugs
Ten days after Vault 106 was sealed, psychoactive drugs were released into the air filtration system. The residents, acting as guinea pigs for this drug testing, were assured that everything was fine when some of them began exhibiting strange neurological symptoms. Eventually, all of them succumbed to the hallucinogenic and went insane. The fate of the Overseer, Dr. Albert Leris, who was aware of the Vault's true purpose, is unknown.
When the player enters this Vault in-game, the drugs are still being pumped through the air system, causing the player's vision to blur. Walking deeper into the drug-filled Vault, you'll see various hallucinations, as well as confront Dwellers who are very much real. Poisoned by the drugs, they'll attack on sight.
6 Vault 36 - (Not So) Picky Eaters
While not featured in-game, Vault 36 is featured in the Fallout Bible, a collection of lore written by one of the games' designers, Chris Avellone. However, Avellone has since confirmed that the Fallout Bible is no longer canon.
Most Vaults were stocked with provisions to last several decades at least. They were also equipped with food synthesizers that would distribute food as needed. However, Vault 36 was intentionally given a food extruder that only provided a sickening, watery gruel, lacking in most essential nutrients. The fate of this Vault is unknown, though one possibility is that the Dwellers died due to lack of nutrition. Another is that they resorted to cannibalism.
5 Vault 55 And 56 - Death By Boredom
The experiment of Vault 55 was to deny the Dwellers all forms of entertainment. Stuck underground with nothing to do, it is likely the residents either found another way to occupy themselves or were slowly driven insane. Vault 56 was a kind of spin-off of 55. All entertainment was removed, save tapes of one particularly bad comedian. The hypothesis of observing sociologists was that Vault 56 would fail before 55. For 56, another theory involves the creation of a cult around the bad comedian, where puns are a form of prayer. A similar idea says that the community becomes a haven of performers, first attempting to one-up the comedian, then each other. It's hard to imagine what those in 55 got up to, but pondering the possibilities is sure to keep you entertained.
4 Vault 34 - Ghoulish Nightmare
Vault 34 was intentionally overstocked with weapons and ammo, and included several recreational facilities at the cost of real living space. After the first several decades, the Vault became overpopulated and the unrestricted weapons access resulted in small bouts of violence. Some residents started demanding reproductive restrictions, leading to more conflict. The Overseer eventually cut off access to the armory, causing a small faction to leave the Vault and settle elsewhere.
The Vault's infrastructure soon began to decay, with radiation leakage becoming a serious problem. A reactor technician, Chris Haversam, believed he was mutating into a ghoul and fled the Vault. Ironically, he was not sick at all, though he was the only one who could've stopped the radiation leak. Soon after, the residents became trapped inside after a bomb blast caused the doors to automatically seal, and one by one mutated into sickening ghouls, retaining their mental state while their bodies rotted away.
3 Vault 92 - Artists Gone Mad
Access to Vault 92 was granted to the world's most talented musicians, under the premise that they would help preserve the arts during the war. The actual experiment behind the Vault involved subliminal messaging through white noise. The idea was to create "super soldiers", combatants who were more aggressive and powerful, yet followed orders with unwavering loyalty. Only part of this goal was reached. While the residents did develop the expected aggression, they proved to be uncontrollable killing machines. One subject physically tore apart three people before 23 gunshots took him down. The Overseer was nonetheless delighted by these results and continued the experiment. More and more people went crazy and killed their fellow Dwellers, eventually causing the Vault to fail. If the player enters this vault, they will encounter both crazy musicians and mirelurks, monsters from the wasteland who had gained access to the Vault.
2 Vault 42 - Dying Of The Light
Normal in almost every other respect, the experiment of Vault 42 involved providing light bulbs no stronger than 40 watts. The anxiety of living underground, coupled with a constant, looming darkness, no doubt had adverse effects on the Dwellers' mental state. The complete lack of sunlight would destabilize their circadian rhythm, though possibly allow them to adapt to low-light conditions. Such was likely the intention of the Vault's designers, hoping to observe the effects of unwavering darkness on a person's mind and body. One can only imagine how those living in Vault 42 would react to sunlight when, and if, the time comes when the Vault is finally programming to open. Their remaining senses heightened, but their bodies unfamiliar with the wasteland environment, it is doubtful that they'd survive.
1 Vault 11 - Democratic Sacrifice
Perhaps the most fascinating social experiment is that of Vault 11. The Dwellers were forced by a computer to elect an Overseer, who would serve one term before being sacrificed. According to the computer, refusal to elect meant everyone would die.
Eventually, several voting blocs emerged, creating a powerful entity that blackmailed other residents into doing their bidding, lest they be voted as the next sacrifice. One such victim was Katherine Stone, whose husband was voted for despite her compliance with the bloc's abhorrent demands. She got her revenge by killing members of the bloc, and, confessing to her crimes, was quickly voted in as the next Overseer/Sacrifice. She used her power to change the voting system to random selection. This led to a bloody coup, leaving only five people alive.
These final five refused to send any more sacrifices and prepared to commit suicide. However, as a response to their defiance, the computer revealed that this was the goal of the experiment, and that their refusal earned them their freedom. Horrified by this shocking truth, they killed themselves anyway, leaving the player to discover their story when they enter the now-empty Vault.