The Unabomber. The Atlanta Olympics bomber. The other Oklahoma City bomber. Those could be Boston Marathon bomber Dzhokhar Tsarnaev's prison-mates if he's spared the death penalty. The prison where he could spend the rest of his life is the most restrictive in the nation, a "supermax" facility that's been called a clean version of hell. The prison — officially known as United States Penitentiary Administrative Maximum Facility, or ADX — is set in the Colorado Rockies' foothills, and its 490 cells house America's most dangerous criminals, from traitors to terrorists. If Tsarnaev winds up there, his contact with other people will likely be severely curtailed, and he'll be housed in a special unit for terrorists. But even for its general population, the prison's isolation is extreme: Prisoners spend about 23 hours a day in solitary, in 12-by-7-foot cells where meals are served through a slot in the door. Just last year, Amnesty International said the prison violated international humane treatment standards.