Hydro-Base is a facility designed as a secure replacement for the incarceration of super-villains that was brought online after the 42 super-prison proved untenable. It has myriad sub-levels under the surface of the ocean. The north sub-basement is the armory, where the super-weapons of incarcerated villains are kept.
Hydro-base is operated by a mixture of SHIELD and SWORD staff, as the facility contains alien incarcerants like the Skrulls and the Kree as well as the more familiar terrestrial villains.