Although Fallout's iconic vaults were billed to the public as impenetrable bastions meant to preserve the basis of American society in the face of nuclear fire, the reality of the situation was just a little bit more complicated than that. Vaults were certainly more than capable of protecting and providing for their inhabitants after the bombs fell, but that purpose was practically secondary to their more nefarious ends.
The majority of the vaults were used to perform various social and scientific experiments on the populations taking refuge within. Some of these studies were relatively tame, if not altruistic in nature. Others, however, ranged from incredibly bizarre to outright cruel. To outline the lengths that Vault-Tec went to in producing these experiments, we've assembled the ten most cruel and disturbing experiments that Vault-Tec ever cooked up for Project Safehouse.
10 Vault 27
Vault 27 actually has yet to appear in any of the actual Fallout titles, only bearing a mention in Chris Avellone's Fallout Bible, a collection of documents providing a wealth of information on the Fallout universe. Although these documents are no longer reliable as "official" Fallout canon, they still provide a lot of interesting information and insight into the Fallout world.
The experiment that was attached to Vault 27 is a relatively straightforward one; it was purposefully overcrowded by two times its maximum capacity. It isn't difficult to imagine the immediate impact of such drastic overpopulation, but, suffice it to say the specifics of the results are uncomfortable to visualize.
9 Vault 19
This vault can be explored during the events of Fallout: New Vegas, where it has since been taken over by the Powder Gangers. But, long before they arrived, its original purpose was to test the impact of subliminal messaging on its inhabitants.
Upon arrival, the residents were segregated into two groups and assigned one of two colors: blue, or red. Afterward, the vault exposed them to various stimuli that were intended to provoke feelings of paranoia and distrust. Terminal entries within the vault indicated that these subliminal messages were very effective and that the mutual distrust between its divided population was rapidly approaching a violent climax.
8 Vault 12
In what is perhaps Vault-Tec's most straightforwardly dishonest experiment, Vault 12 was designed with a purposefully faulty vault door that would not seal properly, allowing radiation to seep into it so that its impact on the human body could be studied.
This would naturally result in the majority of the vault's population becoming ghouls. They would eventually emerge to found the ghoul haven of Necropolis, which the Vault Dweller can explore over the course of the original Fallout title.
7 Vault 75
The experiment attached to Vault 75 is a solid contender for the absolute cruelest experiment ever carried out by Vault-Tec. Situated beneath Malden Middle School in the Commonwealth of Fallout 4, its stated purpose was to protect the school's students and their families in the event of a nuclear attack.
Upon entry, the students' parents were separated from their children and were summarily executed by the security staff under the pretense that they were undergoing "orientation." The children were then subjected to rigorous physical and mental conditioning. Those that did not achieve satisfactory results were disposed of once they reached the age of eighteen, and those that did were "harvested" for their superior genes.
6 Vault 22
The experimentation carried out in Vault 22 wasn't necessarily disturbing at its core. It was designed to sustain life purely by way of "green" living, keeping its inhabitants alive by virtue of the plant life that was grown and cared for within. The results, however, are where things start to get dark.
A certain, specialized fungus was used to keep pests from ruining their crops. Unbeknownst to them, however, were the mutagenic properties of the fungus. Once the spores took root in human hosts, they would gradually transform them into spore carriers, deadly and aggressively cannibalistic mutants that have claimed many lives since.
5 Vault 34
This vault is perhaps most famously remembered as the ancestral home of the Boomers faction in Fallout: New Vegas, where they resided before establishing their community at the Nellis Air Force base. It turns out that their affinity for weaponry has some deep and disturbing roots there.
Vault 34 was purposefully designed with cramped living conditions as well as an absurdly overstocked armory full of weapons and ammunition. The cramped vault would soon become overpopulated, and the residents would continuously request access to the armory in order to protect themselves, which the Overseer would deny. This would eventually result in violent rioting.
4 Vault 106
Vault 106 was designed to simply begin pumping psychoactive drugs into its air filtration system a mere ten days after the door had sealed itself shut. Perfectly aware of the vault's purpose, the Overseer directed security personnel to dismiss any complaints resulting from this.
The Lone Wanderer can explore this vault during Fallout 3 and will quickly come to realize that these drugs are still being filtered into the vault when they begin experiencing vivid and violent hallucinations. The few survivors left inside have been driven completely insane by consistent exposure to them and will attack anyone on sight.
3 Vault 81
The conclusion of Vault 81's experimentation is a true rarity, as the Overseer realized the inhumanity of its purpose and put an end to it before any lasting harm came to its general population. In fact, the Sole Survivor can explore Vault 81 in Fallout 4, meeting its population of relatively sane and decent inhabitants.
Originally, the vault's purpose was to expose the population to an array of deadly diseases and illnesses, while a science team operated from a secret locale to develop and test various cures on the subjects without their knowledge.
2 Vault 95
Vault 95's stated mission almost seems altruistic in nature. It was to serve as a rehabilitation clinic for chem addicts, allowing them to overcome their various addictions, even as the outside world collapsed under a volley of nuclear missiles. However, its true purpose was much less than kind.
Although its detox program was a great success, a massive stash of chems and alcohol was hidden inside of the vault, and a planted resident would "unearth" this cache after five years' time. Predictably, the staged discovery of this cache precipitated the vault's program unraveling entirely, and its population plunged into chem-fueled anarchy.
1 Vault 11
Of all the cruel experiments that Vault-Tec executed on unsuspecting victims, the one taking place at Vault 11 has to be the worst. Vault 11's inhabitants were instructed to select one of their own as a human sacrifice each year. If they refused, they were told that the vault would shut off its life support systems, and the entire population would die.
In reality, should the vault's residents refuse to sacrifice someone, the vault would provide an automated message congratulating them as "a shining example of humanity." By the time that they couldn't bear to sacrifice anyone else and bravely refused, its population had been whittled down to a mere five people. Unable to cope with their guilt after this brutal realization, four of those five people elected to commit suicide.