potentially dangerous tap. Rooms may also be fitted with polished steel mirrors bolted to the wall, an electric light that can be shut off only remotely, a radio, and a television that shows recreational, educational, and religious programming. The 4-inch-by-4-foot (10 cm × 1.2 m) windows are designed to prevent inmates from knowing their specific location within the complex. They can see only the sky and roof through them, so it is virtually impossible to plan an escape. Inmates exercise in a concrete pit resembling an empty swimming pool, also designed to prevent them from knowing their location in the facility. The pit is large enough only for a prisoner to walk ten steps in a straight line or thirty-one steps in a circle. Correctional officers generally deliver food to the cells. Inmates transferred to USP ADX Florence from other prisons may be allowed to eat in a shared dining room. The prison as a whole contains a multitude of motion detectors, cameras, and 1,400 remote-controlled steel doors. Officers in the prison's control center monitor inmates twenty-four hours a day and can activate a "panic button", which immediately closes every door in the facility, should an escape attempt be suspected. Pressure pads and razor-wire fences surround the perimeter, which is patrolled by heavily armed officers. The facility houses inmates at six differing security levels: General Population Units ("Delta", "Echo", "Fox", and "Golf" Units), the Special Housing Unit (SHU), the Special Security Unit ("H" Unit), the Control Unit, Intermediate/Transitional Units ("Kilo" and "Joker" Units), and Range 13. Many of the security levels at ADX have special purposes or missions for the inmates who occupy them. The Control Unit houses inmates who have committed serious conduct violations or acts of violence at other institutions. It also houses high-level members of organizations deemed as threats, such as prison gangs. "H" Unit houses inmates who are members of terror groups so designated by the Department of Justice or who have had special administrative measures (SAMs) placed on them. Range 13 is a special four-cell wing within the Special Housing Unit for inmates in need of the tightest control. As of 2022, the only inmates publicly known to have been incarcerated in this unit are Thomas Silverstein and Ramzi Yousef. The two Intermediate Units house "step-down" inmates, who can earn transfer to another institution if they remain incident free while housed in the unit. This