Supermax.

Described by Olympic bomber Eric Rudolph:

"It is a closed-off world designed to isolate inmates from social and environmental stimuli, with the ultimate purpose of causing mental illness and chronic physical conditions such as diabetes, heart disease and arthritis," he wrote in one letter to The Gazette of Colorado Springs.

Rudolph wrote that he spends 23 hours a day in his 7-by-12-foot cell, his only exercise confined to an enclosed area he described as a "large empty swimming pool" divided into "dog-kennel style cages."

"Using solitary confinement, Supermax is designed to inflict as much misery and pain as is constitutionally permissible," he wrote in a letter.

Interesting that he (very nearly) concedes it's constitutional.