When inmates arrive at the United States Penitentiary Administrative-Maximum Facility in Florence, Colorado, it immediately becomes clear: ADX, the nation's most secure Supermax prison, is built to cut them off from the world.

Now that Boston Marathon bomber Dzhokhar Tsarnaev, 21, has been formally sentenced to death , his final destination will be in the hands of the Federal Bureau of Prisons. On Thursday, he was transferred to the U.S. Penitentiary in Florence, one of several correctional facilities in the city, including Supermax.

The worst of the worst in America's vast prison network are delivered to ADX, the "Alcatraz of the Rockies," in buses, special vehicles, even Black Hawk helicopters.

Heavily armed patrols cruise the sprawling complex. A dozen imposing gun towers rise above squat brick buildings. Walls topped with razor wire partially block the snow-capped mountains.

A picture of Tsarnaev from his apparent profile on VKontakte, a Russian social network similar to Facebook.

A picture of Tsarnaev from his apparent profile on VKontakte, a Russian social network similar to Facebook.

A still of the suspects from footage released by the FBI after the bombing.

A still of the suspects from footage released by the FBI after the bombing.

Tsarnaev was seen on this convenience store surveillance video that was released by the Boston Police Department.

Tsarnaev was seen on this convenience store surveillance video that was released by the Boston Police Department.

An image posted to the social sharing website Reddit purportedly shows Dzhokhar Tsarnaev being detained by law enforcement officers.

An image posted to the social sharing website Reddit purportedly shows Dzhokhar Tsarnaev being detained by law enforcement officers.

Tsarnaev stands in court, flanked by his lawyers, in this sketch from July 2013.

Tsarnaev stands in court, flanked by his lawyers, in this sketch from July 2013.

The August 2013 cover of Rolling Stone featured Tsarnaev and sparked a backlash against the magazine.

Dzhokhar Tsarnaev was found on April 19, 2013, in a boat that was dry-docked in the backyard of a Watertown home. He was covered in blood from bullet wounds.

Dzhokhar Tsarnaev was found on April 19, 2013, in a boat that was dry-docked in the backyard of a Watertown home. He was covered in blood from bullet wounds.

On April 18, 2013, the FBI released photos and videos of two suspects and asked the public to help identify them.

On April 18, 2013, the FBI released photos and videos of two suspects and asked the public to help identify them.

Dzhokhar Tsarnaev was arrested on April 19, 2013, after a massive manhunt. An overnight shootout with police killed the other suspect -- Tsarnaev's 26-year-old brother, Tamerlan. A jury condemned Tsarnaev to death on Friday, May 15, for his role in killing four people and wounding hundreds more.

Dzhokhar Tsarnaev was arrested on April 19, 2013, after a massive manhunt. An overnight shootout with police killed the other suspect -- Tsarnaev's 26-year-old brother, Tamerlan. A jury condemned Tsarnaev to death on Friday, May 15, for his role in killing four people and wounding hundreds more.

"As soon as they come through the door ... you see it in their faces," former ADX warden Robert Hood said. "That's when it really hits you. You're looking at the beauty of the Rocky Mountains in the backdrop. When you get inside, that is the last time you will ever see it."

"The Supermax is life after death," said Hood, who served as ADX warden from 2002 to 2005. "It's long term. ... In my opinion, it's far much worse than death."

'The architecture is the control'

Many of the more than 400 inmates spend as much as 23 hours a day alone in 7-by-12-foot concrete cells. Meals are slid through small holes in the doors. Bed is a concrete slab dressed with a thin mattress and blankets.

A single window about 42 inches high and 4 inches wide allows some natural light but is made so prisoners cannot see beyond the building. Cells have unmovable stools and desks made of concrete. Solid walls prevent prisoners from seeing other cells or having direct contact with other inmates.

"The architecture of the building is the control," Hood said.

"You're designing it so the inmates can't see the sky. Intentionally. You're putting up wires so helicopters can't land."

Inmates have little contact outside of guards and prison staff. They must wear leg irons, handcuffs and stomach chains when taken outside their cells -- and be escorted by guards. A recreation hour is allowed in an outdoor cage slightly larger than the prison cells. Inside the cage, only the sky is visible.

"You're passing hundreds, hundreds of cameras as metal doors are sliding open and closed," Hood said.

Life in the H-Unit

Some cells have radios and black-and-white televisions offering religious, educational and general interest programs.

Mail and conversations are monitored at all times, the current ADX warden, John Oliver, testified at Tsarnaev's sentencing hearing. Inmates at some point may be able to get prison jobs, such as cleaning showers, or move into the general population, Oliver said.

But the level of freedom a prisoner such as Tsarnaev would enjoy is ultimately determined only by the Justice Department and the agencies that investigated and prosecuted him, not the prison staff.

Tsarnaev will likely join other terrorists in the Special Security Unit, also called the H-Unit. These cells are reserved for inmates with DOJ-imposed Special Administrative Measures intended to strictly limit all communications with the outside world.

Only members of a prisoner's legal team and immediate family are permitted to visit. Prisoners sit on the other side of a glass window. They speak over telephones. All personal conversations are monitored, but legal conversations and correspondence with attorneys are considered privileged and private.

Earning 'the right to go to Supermax'

"ADX itself has ... become almost entirely a 'lock-down' facility in which prisoners are locked in solitary cells for all but a few hours a week," Amnesty International said in a 2014 report titled "Entombed: Isolation in the U.S. federal prison system."

The Supermax is home to the prison system's most violent inmates as well as convicted terrorists.

"They've been in jail. They've been in prison. They've killed staff. They've killed a visitor," Hood said. "They've earned, if you will, the right to go to Supermax. ... These are terrorists. These are disruptive gang members. They're spies."

A 2012 class action suit against the Bureau of Prisons said "years of isolation, with no direct, unrestrained contact with other human beings" leave some ADX inmates -- particularly those with serious mental illness -- with "a fundamental loss of even basic social skills and adaptive behaviors." They "predictably find themselves paranoid about the motives and intentions of others."

"Once placed into unrestrained contact with other, similarly impaired and paranoid men, the stress on prisoners -- even those with no mental illness -- can be extreme. Assaults and stabbings are common."

Many ADX prisoners "interminably wail, scream and bang on the walls of their cells," the lawsuit said. "Some mutilate their bodies with razors, shards of glass, sharpened chicken bones, writing utensils, and whatever other objects they can obtain. A number swallow razor blades, nail clippers, parts of radios and televisions, broken glass, and other dangerous objects."

Coping -- or self-destructing?

Some inmates have "delusional conversations with voices they hear in their heads," the court documents said. Others spread feces, other human waste and body fluids throughout their cells or hurl it at correctional officers.

"I do know that when you put a person in a box for 23 hours a day and you tell them that's the rest of your life, that each person has their own coping skills," Hood said.

"When you see a person disrobing, throwing feces at a staff member going by -- is that mental illness? Is that an issue where they're self-destructing?"

At least six prisoners have committed suicide since ADX opened in 1994, the lawsuit said. Most of the suicides involved prisoners hanging themselves with bedsheets.

"Though I know that I want to live and have always been a survivor, I have often wished for death," Thomas Silverstein, confined for more than 30 years in isolation, including nine in ADX, was quoted as saying in the Amnesty International report. "I know, though, that I don't want to die. What I want is a life in prison that I can fill with some meaning."

Laura Rovner, a University of Denver College of Law professor who has represented ADX prisoners, said reports of conditions at the notorious Guantanamo Bay prison in Cuba compare favorably with some conditions at ADX.

"For many people, being confined at ADX in what will amount to a life sentence there really is kind of a form of living death," she said. "It just takes everything away from you. Your existence is limited to the four walls of this small cell and frankly not much else."

The mentally ill and younger inmates are particularly vulnerable, Rovner said.

"This is a person who's going to be vulnerable, who's going to feel the isolation in ways that are more acute," she said of Tsarnaev. "He's presumably going to be alive for a long time. He's looking at spending potentially at least the next 50 years in isolation. It's almost unfathomable."

Prisoners in the H-Unit rarely have access to less-restricted general population units, according to Amnesty International. In 2008, the prison instituted a step-down program for the H-Unit consisting of three phases lasting a minimum of one year -- with each step providing limited privileges.

"If you're the Unabomber and you have an advanced degree ... and know multiple languages, you're going to sit there and read most of the day," Hood said of Kaczynski, who has been described by acquaintances as brilliant.

"But many of the inmates do not have the coping skills. They don't have the reading ability. They don't have the ability to be litigious. So there's no outlet; that's most likely the inmate who is going to throw feces at you."

Prisoner advocates have found that some inmates, despite good conduct, spend years in H-Unit without progressing to the next phase because the Special Administrative Measures were not modified.

The World Trade Center terrorist

Ramzi Yousef is serving two life sentences plus 240 years for his role in two terrorist attacks, including the 1993 World Trade Center bombing that killed six people. He has spent more than 15 years in solitary confinement. He's held in H-Unit under Special Administrative Measures and has spent more than two years on step 2 of the step-down program, according to Amnesty International.

Yousef, who has had a clear conduct record for at least five years, has worked as an orderly, which allows him out of his cell a few hours a week to clean other cells. Still, he has been denied access to step 3, and his Special Administrative Measures are renewed each year, the rights group said.

Hood described Yousef as civil but said he isn't as personable as mob turncoat "Sammy the Bull" Gravano.

"Here's a guy the first time you meet him, you actually like him," Hood said of the former Gambino crime family enforcer. "You don't like what he did. But you find a likable person, a person you'd want for your next-door neighbor. He's funny. He's appropriate for relations within the prison setting. You actually feel good to see him every day."

Of Yousef, the ex-warden said: "Regardless of the crime ... regardless of what he's been involved in, he's well-trained. He's disciplined. I would be the enemy even though I am the warden. Yet we also had that civility. I would say good morning, how are the staff treating you?"

During the sentencing phase of Tsarnaev's trial, Oliver, the current ADX warden, portrayed the Supermax prison in the best light possible, describing how inmates in the special security units can mail letters, exercise in their cells, talk on the phone for up to 30 minutes a month and even write books.