Smelly Lid Prison

Smelly Lid Prison (臭蓋獄, Kusa futa goku; Viz: The Stink Slammer) is a prison located in an unknown city. This prison is used to hold some of the worst criminals in the world of One-Punch Man.

Features
The prison appears to have a tall gate and tall walls with barbed wires on top. The building is heavily monitored with cameras on different locations with large monitors for the guards to see. For more difficult prisoners like Speed-o'-Sound Sonic, they use a special restraining device designed by the Hero Association. The cells that hold the most difficult prisoners is behind a thick, barricaded door. Inside are the most difficult offenders, who can use their strength to try open iron bars or can use everyday objects as weapons and poison. The inside of the walls are made of steel and there are no windows, and only Puri-Puri Prisoner can break through the walls.

As part of the Monster Association raid, some prisoners have been transformed into monsters by Nyan.