Vault

The Vaults are a series of massive underground fallout shelters developed by Vault-Tec in the mid-21st century. One of these is Vault 13, the starting point of Fallout.

Initially it appeared that the Vaults were designed to save the population of the US; however, there were not nearly ebough Vaults for this purpose. The true, sinister reason is that these 'shelters' were in fact a series of sociological experiments conducted by the Enclave. Each Vault was set up with different conditions, and then observed to see how the dwellers within coped with the pressure of isolation.

From the Fallout Bible: