Tim Cain, one of the original creators of the Fallout series, has revealed on his YouTube channel (watch the full video here) that the vaults initially had a much grander purpose.
They were initially meant to send people to space, he said, hoping they would find a new planet to live on. This, of course, harkens the upcoming Starfield's plot to mind, where humans have already begun space exploration and colonizing planets.
In the Fallout series, the entire world has been affected by nuclear missiles, leaving some people and creatures to mutate while those lucky enough not to be affected by the radiation fend for themselves.
The nuclear war was something that the Enclave (Fallout faction) and the U.S. government saw coming, so to prepare for this, they decided to take their "best and brightest" to space in hopes of finding a new earth. But they realized that this couldn't be done before the bombs dropped, and Vault-Tec thought it could use their vaults as trial runs to help get them to space.
Cain explained that getting the vault dwellers to space would be a "multi-generation" plan because they needed to figure out how to "grow plants really well in an enclosed environment" and ensure that the water being used was safe to consume.
In short, the vaults were originally made to solve the sorts of problems that they may encounter while in space. This makes sense because in games like Fallout 3, New Vegas, and even Fallout 4, you can find vault horror stories of people being cloned, plants coming to life, and so much more.
"Every vault was in some sense a test," Cain said.
Even though this idea was conceived after Fallout 1 was completed, Vault 13 fits into this concept too, as the vault wasn't designed to work and "they wanted to
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