The locations in the post-nuclear United States of America form the backdrop in which the events of the Fallout series occur. Many have diverse environments where anything can happen. Such as a special encounter, or just a skirmish with raiders. The environments and places have been changed over the course of the many games, as the settings have always been in different places, and always brought new surprises with them.
There were a total of 122 Vaults that were known to the public, and this number may not include any of the secret Vaults that the pre-War American government could have constructed for private purposes of protecting high-ranking officials. Most of the Vaults were actually created for testing some sort of social, or emotional flaw and how they would react to it, and only about 17 of the Vaults were actually meant for people to live normal lives in the Vault, and not to be exposed to carnivorous plants (Vault 22), or the door not meant to close all the way, thus exposing the residents to lethal amounts of radiation (Vault 12).