The Vault

The Vault, also referred to as The Box, is a massive floating structure created by the Combine before the completion of the Citadel. Located deep within the Quarantine Zone, an area of City 17 that is under lockdown and separated from the rest of the city, it is an important and highly kept secret structure to the Combine, and the purpose of the Vault is one of the major plot points of Half-Life: Alyx.