Vault

The Vaults are a series of massive underground fallout shelters developed by Vault-Tec in the mid-21st century.

The vaults initially appeared as a haven for the endangered U.S. population. However, there were not nearly enough Vaults to achieve the task. The true reason for these 'shelters' was that in fact they were a series of sociological experiments conducted by a group known as the Enclave. Each Vault was set up with different conditions and observed to see how the dwellers within coped with varying conditions and the pressures of isolation.
("Vault", The Vault - a Fallout Wiki)