Texas has the largest prison system in America, with more than 150,000 prisoners behind bars. The headquarters of the state’s Department of Criminal Justice is in Huntsville, a small, conservative town that’s home to nine state prisons. In the center of Huntsville is the Walls Unit. The oldest prison in Texas, it has gained notoriety in recent years as the location of Texas’ famously-frequent executions. But another, less-known function of the Walls Unit is mustering out the Texas system’s prisoners: every day, more than 150 men are processed, paroled and released.