The prison is known as Supermax: the most-secure federal prison in the United States. Located in the state of Colorado, it houses some of the world's most notorious criminals, including Mexican drug lord Joaquin "El Chapo" Guzman. 

But the prison is also controversial. 

CGTN's Hendrik Sybrandy  explains why.

Florence, Colorado is an old mining community that prides itself on selling antiques. 

"A vibrant, fun place to live," said City Councilman Larry Baker. "It truly is."

But just outside the town lies a prison some have called a "clean version of hell." This is ADX, or Supermax, where those deemed the most dangerous prisoners in the country, the so-called worst of the worst, are kept. 

"The level of isolation there is dramatic," said Laura Rovner with the University of Denver's Sturm College of Law. "It's just an incredibly grim place."

Rovner has represented inmates at ADX, which is home to some 400 prisoners, including notorious criminals like the World Trade Center bomber, the Shoe Bomber, the Unabomber, the Olympic Park Bomber, the Boston Marathon bomber and this longtime Soviet spy. 

Drug kingpin El Chapo escaped twice from Mexican prisons but is unlikely to repeat the feat at this ultra-secure facility. We got a glimpse of the place shortly after it opened in 1994. The concrete cells are sparse and small. Most inmates are alone for 23 hours a day. 

"I wish that a lot of young people who might be thinking about crime could stand in an institution like this and understand what it's like to hear a jail cell clang behind them and know that they'll be here for a very long time," said then, US Attorney General Janet Reno. 

"The horror of it really lurks beneath the surface," said Rovner. 

She runs the University of Denver's Civil Rights Clinic, which has fought to loosen the restrictive conditions at ADX over the years, with limited success. She said the isolation of prisoners is extreme. 

"Everything that we know now is that solitary confinement is incredibly destructive to the human spirit," she said. "People need human contact, people need environmental stimulation. You need things to do to occupy your brain." 

"The studies that been done show that's it's tremendous, irreparable, damage to these people psychologically," said criminal justice expert Mark Pogrebin with the University of Colorado-Denver. 

Pogrebin and others said ADX was built with control and punishment in mind, not rehabilitation, even though many inmates there are eventually released. 

The facility is as difficult to get into as it is for prisoners to get out. Securing interviews with prisoners is very difficult. Recording video of the inside or getting the warden to talk is nearly impossible. 

"There aren't a lot of victories there for civil liberties," said journalist Alan Prendergast. 

Prendergast has covered the prison extensively and said the facility has become much less transparent in recent years. People on the outside, he adds, don't lose much sleep over the condition of the prisoners there. 

"They're either these high-profile and unsympathetic inmates or they're people we've never heard of and we could probably care less about," he said. 

"We're paying for these damn places," said Pogrebin. "I say if you're doing the right thing, open it up, let the press in, let the public see. But they don't do it and I know why. They don't want people to see the results and what happens to people when they're in there that long." 

A court settlement several years ago resulted in major changes in the way mentally ill prisoners at ADX are treated. Rovner said that was a start but many problems remain. 

"I think for me, the question is what do we owe each other as fellow human beings," she said. 

Here in this beautiful setting that most prisoners will never see, life is hard. It's now  El Chapo's new address, and likely will be for many years to come.