Pelican Bay State Prison is a supermax security facility located near Crescent City, California. While housing a large "General Population" group of prisoners, it also features an X-shaped cluster of white buildings (separated by electrified concertina wire fencing), known as the Security Housing Unit. This is where prisoners identified as gang members, prisoners with a history of violence, crimes or serious rules violations within prison, and other prisoners presenting serious management concerns are incarcerated. The Pelican Bay SHU was one of the first such facilities in modern American history explicitly planned and built as a control-unit facility. SHU inmates are held in isolation 22.5 hours per day in their undecorated cell and one hour alone in a small indoor exercise yard. Radios and TVs are allowed.
When convicted of treason (and possibly terrorism), Charles Fischer was sentenced to life in incarceration in the SHU. It would, however, be his saving grace. It is likely that Skynet "rescued" other prisoners from this location and either enslaved them or employed them as Grays. In the case of the latter, those sentenced to life in solitary confinement would likely be the perfect candidates to be Skynet collaborators.