Among the many areas that can be discovered in Fallout 4, players can encounter several Vaults to explore - many of which have a hidden purpose outside of just keeping residents save from radiation. Here's a couple of Vault-Tec's weirdest experiments that can be found in Fallout 4.
When they were initially constructed, Vaults in the Fallout series were intended to provide shelter in the event of nuclear war. However, the company responsible for their construction, Vault-Tec, also had additional goals in mind when they were constructed. Each Vault functioned as a social or scientific experiment. Vault residents were mostly unaware of this information. Although many Vaults failed because of the horrific tests the dwellers were put through, their history can be found in the game and some are even still fully-functioning.
Fallout 4 only features a small handful of Vaults, but each one contains a rich backstory. The game even begins in a Vault, similarly to some other games in the franchise such as Fallout 3. Several of the game's DLC also add additional Vaults into Fallout 4. By uncovering the lore of the Vaults, usually found on terminals located throughout the area, players can discover the disturbing and sometimes downright absurd experiments conducted by Vault-Tec across the Fallout series.
Weirdest Fallout 4 Vault: Vault 75
Vault 75 can be found underneath Malden Middle School in Fallout 4. It was initially presented as a safe place for the school's children to take cover in the potential event of nuclear war. When the time came to make use of the Vault, the designated families entered and were promptly separated from their children and executed. The true purpose of the Vault was to genetically enhance the children who had entered it, eventually harvesting them for their good genes if they had been strong or intelligent enough to survive the mental and physical tests they were put through. When the player character finds this Vault in Fallout 4, it has already been opened and emptied.
Weirdest Fallout 4 Vault: Vault 81
A recurring feature when it comes to the Vaults is Vault-Tec's lack of concern for the safety of their residents, and many Vaults in various Fallout games highlight this. Vault 81 is another example, and can be found east of Forest Grove Marsh. Unlike most of the other Vaults in the game, Vault 81 is still active when the player encounters it. The goal of the Vault was to create a single universal cure for every possible disease, and residents were used as test subjects for this endeavor. During their stay in the Vault they were intended to be subjected to numerous diseases and cures. Fortunately, the first Vault overseer managed to prevent this outcome, and the Vault's residents can be found alive and healthy if encountered in Fallout 4.