Vault 12 was a huge facility, able to support over a thousand residents. It was also one of the best stocked of all the vaults, with an abundance of amenities, both recreational and practical.
Unfortunately for the inhabitants though, the door was never closed fully. Thanks to the lack of a sealed entrance, the entire vault was exposed to the outside world. This not only meant that the insides became weathered and unreliable, but also that the inhabitants were slowly exposed to radiation. Vault 12 was also forced into by outsiders after they realised the door wasn’t sealed, leading to extensive overpopulation. Despite being relatively unaffected by the explosive blasts themselves, the residents of the vault succumbed to the creeping radiation, turning them into ghouls. Ultimately, this led to the founding of the ghoul-town Necropolis, an important location in the original Fallout game.