42 (Earth-8543)

42 is a special prison located in the Phantom Zone that was designed by Reed Richards, Tony Stark, Bruce Wayne, and Hank Pym as their fourteenth second idea to make the world safer from supervillains.

History
The prison's only entrance and exit is through a portal that resides within the Baxter Building, which in turn is guarded by elite S.H.I.E.L.D. Mandroids.

The inside of the prison was formerly guarded by Ultron units that Hank Pym designed to rehabiliate its prisoners. After they were used to even the odds against Kang all but one Ultron remained, so they were replaced by S.H.I.E.L.D. agents.

42 was ultimately abandoned after it was discovered that there lied life within the Phantom Zone that sought to use the portal to invade Earth and crush all positive matter. The Hydro-Base was subsequently built to house all remaining prisoners.