Alcatraz is the oldest and largest ‘Super-Max’ prison in the US, meaning that it has the capability to hold metahuman prisoners with exotic powers. There are two sections to the prison: the aboveground facilities, which include all the office and infrastructure support spaces, power generation, docks and gunship landing facilities. It operates as a joint US-UN paramilitary facility, held both by SIGMA and ATLAS. Metacriminals that are not deemed a flight risk are housed in a special section; they participate in work-release programs and otherwise enjoy the benefits of a ‘normal’ high-security facility. The infirmary is well-equipped to deal with metahuman medical needs.
The below-ground facilities are known as Lockdown and are maintained by ATLAS personnel, since many of the incarcerated are of multiple national origins, housed here at the request of their home nations (many of which do not have similar facilities). To say that this is a political hot potato would be a severe understatement. These facilities house the most dangerous criminals on Earth in power-nullification cells or in unique restraints.