California's Pelican Bay Prison is the most notorious state penitentiary in America. Designed and built as a "supermax" facility, it's been used for nearly 30 years to lock away inmates considered the most dangerous.

Pelican Bay's security housing unit - known as "the SHU" - is solitary confinement by another name, and inmates and their advocates have long denounced it as state-sanctioned torture.

The people who run California's prisons defended their approach for decades. But as we first reported last October, they are now at the center of a reform movement that is dramatically reducing the use of solitary confinement across the country and at Pelican Bay.

Oprah Winfrey goes inside Pelican Bay's Security Housing Unit – known as "the SHU." CBS News

OPRAH WINFREY: Hi there.

ARON FRANKLIN: Hi, nice to meet you.

OPRAH WINFREY: Nice to meet you. I'm Oprah.

ARON FRANKLIN: Hi, I'm Aron. I-- I know who you are.

OPRAH WINFREY: Oh, Hi Aron.

On the other side of that steel mesh, inmate Aron Franklin is serving part of his 50 years to life sentence.

OPRAH WINFREY: What did you get 50 years to life for?

SHU pod CBS News

ARON FRANKLIN: For a murder.

OPRAH WINFREY: Murder.

ARON FRANKLIN: Yeah.

"We are, most of us, going to be getting out. And it would behoove the public to begin to facilitate a healing, you know? And the healing can start with, you know a basic dignity in how we're treated."

It was the murder of a fellow gang member in San Diego. But crimes you commit on the outside don't get you sent to the Pelican Bay SHU. It is reserved for offenses committed once you're in prison.

OPRAH WINFREY: Why were you brought here? Can you tell me?

ARON FRANKLIN: Just a little misunderstanding on the yard.

That "little misunderstanding" was an attack on another inmate with a weapon, and it earned him a year in solitary confinement. Franklin is in what's known as a SHU "pod;" eight tiny cells, four up and four down, all facing the same blank wall across the way.

DANNY MURILLO: It was created to break me, mentally, physically and spiritually.

Danny Murillo, Troy Williams, and Steve Czifra all went to prison as teenagers. They were sent to the Pelican Bay SHU for what happened after they were behind bars. CBS News

Danny Murillo, Troy Williams, and Steve Czifra all went to prison as teenagers. They were sent to the Pelican Bay SHU for what happened after they were behind bars. Steve spit on a prison guard, Troy was part of a riot at another facility, and Danny was accused of being in a prison gang.

OPRAH WINFREY: Do you remember the first day you pulled up to the SHU, taking that long bus ride, getting off the bus and seeing the place?

DANNY MURILLO: It's a big white building with a small little door. My imagination was-- a human slaughterhouse. People just going into a human slaughterhouse.

OPRAH WINFREY: What did you think, Steve?

STEVE CZIFRA: It was a modern-day dungeon. There was-- I had never seen anything like it.

OPRAH WINFREY: This is-- the message is, "You're not gettin' outta here"?

STEVE CZIFRA: The message is you-- you're screwed.

All three ultimately did get out of the SHU and out of prison.

OPRAH WINFREY: I think the feeling on the part of a lot of folks is that you committed a crime, regardless of what age you were. You got locked up. You deserve to be there. Can you tell me why we should care?

TROY WILLIAMS: We are, most of us, going to be getting out. And it would behoove the public to begin to facilitate a healing, you know? And the healing can start with, you know a basic dignity in how we're treated.

Here inside the Pelican Bay SHU, an inmate would spend up to 22.5 hours a day in this cell, which is basically the size of a small-parking space. It's like a windowless box with a sink and a toilet. Not just for days at a time, sometimes years, and even decades at a time, in this room, alone.

Oprah Winfrey in an empty SHU cell at Pelican Bay State Prison. CBS News

Most days, the only time a prisoner leaves his cell is to go to "the yard," a slightly less tiny concrete box at the end of the pod, for 90 minutes of exercise.

OPRAH WINFREY: So, this is it. This is the yard. This is the extent of the yard? This--

SCOTT KERNAN: Yes. This is it.

OPRAH WINFREY: Yeah. OK. Well, I wouldn't exactly call it a yard.

We visited the yard with Scott Kernan, who runs the California Department of Corrections and Rehabilitation.

OPRAH WINFREY: So it would just be the inmate alone out here.

SCOTT KERNAN: Correct.

On the rare occasions that an inmate leaves his pod, he first has to strip, push his clothes through a slot to be searched, then put his hands through the same slot to be cuffed. This is the only time a SHU prisoner is ever touched by another human being.

1993 60 MINUTES REPORT EXCERPT:

MIKE WALLACE: They do hard time here in the SHU. Time here is like hard time in no other prison.

When Mike Wallace visited Pelican Bay for 60 Minutes in 1993, prisons across the country had embraced solitary confinement as a tool to combat violence inside their walls; there was a building boom in supermax facilities, and Pelican Bay was a model.

MIKE WALLACE: The State of California that runs it proudly proclaims it's the wave of the future, designed to isolate prisoners who, they insist, are out of control, too violent, too unpredictable to be housed with the run-of-the-mill murderers and rapists.

At its peak in the 90s, Corrections Secretary Scott Kernan says Pelican Bay's SHU held almost 2,000 prisoners.

SCOTT KERNAN: During that period of time-- I witnessed multiple murders, multiple stabbings, lives changed irreparably--

OPRAH WINFREY: Inmates stabbing each other? Stabbing corrections officers-- stabbing--

SCOTT KERNAN: All of it.

Almost all of that violence, Kernan says, was and still is caused by powerful race-based prison gangs.

OPRAH WINFREY: So the gangs rule in prison?

SCOTT KERNAN: They do. The gangs rule.

In an effort to break that rule, California identified gang leaders and enforcers and sent them to Pelican Bay.

OPRAH WINFREY: So the idea was to bring them here and have them in isolation?

SCOTT KERNAN: Have them in isolation and deter their communication. And it worked.

OPRAH WINFREY: So if any inmate was validated as a gang member, he could be held here indefinitely for years or decades?

SCOTT KERNAN: Yes.

CLYDE JACKSON: 24 years, five months and six days I was there.

At California State Prison Solano, Oprah Winfrey interviewed inmate Clyde Jackson who spent 24 years in the Pelican Bay SHU. CBS News

Clyde Jackson was sent to prison at age 17 for kidnapping, rape, robbery, and attempted murder. But it was gang ties that got him sent to pelican bay.

Clyde Jackson: Well, I was sent to Pelican Bay SHU because I was labeled as a validated gang member of the Black Guerilla Family. The design was complete isolation.

Craig Haney: One of the first things they'd say to me was "I am struggling to maintain my sanity and I don't know how to do it."

Craig Haney is a psychology professor at UC Santa Cruz whose studies of Pelican Bay SHU inmates have become central to arguments against the widespread use of solitary confinement.

OPRAH WINFREY: So what was the most striking result of your findings in 1992 after that first study?

CRAIG HANEY: That vast numbers of prisoners were traumatized by the experience. They were suffering, they were living in pain, and many of them were being psychologically damaged by the conditions of their confinement. And-- and at-- at much higher levels than even I anticipated.

CLYDE JACKSON: Your mind becomes diseased, and you start to accept the abnormal as normal.

Clyde Jackson CBS News

OPRAH WINFREY: You must know that there are a lot of people who do not care that you're in isolation five years, 10 years, 24 years. What does it matter that conditions are bad because you've got it coming?

CLYDE JACKSON: Well, there's prison and then there's prison, right? The judge sentenced me to prison. He didn't sent me-- he didn't sentence me to an underground prison.

OPRAH WINFREY: But wasn't the logic that it was a serious and valid response to a very real and dangerous wave of violence from gang members?

CRAIG HANEY: There was no reason to believe that that place was going to effectively address the gang problem that was growing in California. And witness the fact that it hasn't. Pelican Bay and places like it in California have been in operation now for many, many years. Decades. We have the worst prison-gang problem in the United States. So it clearly was not a solution.

You might expect Corrections Secretary Scott Kernan to flatly reject that assertion. He doesn't.

SCOTT KERNAN: That was a policy that was intended to save lives and make prisons safer across the system. It was a mistake, in retrospect, as we look back--

OPRAH WINFREY: But you said earlier it worked?

SCOTT KERNAN: It did work.

OPRAH WINFREY: It worked in reducing crime in the general prison population?

KERNAN: Yes.

OPRAH WINFREY: Why did it not work?

SECRETARY SCOTT KERNAN: It didn't work because of the impact on the offenders.

OPRAH WINFREY: I'm sure you've heard that statement from Justice Anthony Kennedy, who says, "Solitary confinement drives men mad." Does it?

SCOTT KERNAN: I think it does.

Remember, that's not some human rights campaigner saying that…he runs the prison system!

OPRAH WINFREY: Does that make you feel any better that there's an acknowledgement from the state that it was a mistake?

DANNY: It doesn't make me happy. I still been tortured.

TROY: Makes you feel like you've been experimented on, really.

CRAIG HANEY: There was plenty of evidence early on that this was a failed experiment. That it was hurting people.

Pressure for change really began to build in 2011, when SHU inmates organized a series of hunger strikes to draw attention to their plight. They also filed a class-action lawsuit challenging the use of solitary confinement.

Fearing it might lose that suit, the state negotiated a settlement with "prisoner-plaintiffs" in 2015. California agreed to stop holding inmates in solitary for indefinite terms, and to stop sending them to the SHU simply for having gang ties.

OPRAH WINFREY: Now that the settlement has happened and the reforms have taken place what is the difference in the SHU now, versus then?

SCOTT KERNAN: The SHU facility that we are doing this interview in is empty. We emptied the SHUs out.

California's SHUs now hold 80 percent fewer inmates than just a few years ago. Only people like Aron Franklin, whom we met earlier, are still sent to the SHU, for specific infractions and limited terms.

SCOTT KERNAN: I think across the nation, people are looking at how we house restricted offenders and are making changes to that policy.

So if Pelican Bay was once a model for the widespread use of solitary confinement, it's now so empty that Scott Kernan is converting SHU pods to minimum-security units.

OPRAH WINFREY (taking off stab vest): So now we can take these off--

SCOTT KERNAN: Yes.

OPRAH WINFREY: --right? 'Cause we're going to a minimum-security unit.

SCOTT KERNAN: Yes.

OPRAH WINFREY: So-- where there's less fear of being stabbed.

OPRAH WINFREY: Oh, this is very different. Wow.

All the cell doors are open in the converted pods, and prisoners can move around freely.

OPRAH WINFREY: Did you ever think that would happen?

CLYDE JACKSON: No. No. I thought, you know I was under the mindset Pelican Bay would be there for an eternity.

After his own eternity in the Pelican Bay SHU – 24 years -- Clyde Jackson is now in the general population at Solano State Prison, near Sacramento.

OPRAH WINFREY: What was it like the first time you were taken out of the SHU and able to experience the environment?

CLYDE JACKSON: Well, Ms. Winfrey, to be honest with you, I was dizzy. It's like being born again.

At Solano, Jackson, has immersed himself in the rehabilitation programs that are now the focus of California's prison system. The state has gone from 'lock 'em up' to 'fix 'em up.'

CLYDE JACKSON: I'm 54 years old. I'm finally in a position to get my GED.

OPRAH WINFREY: And so you're taking advantage of everything you can?

CLYDE JACKSON: Everything that I can that I missed in the past.

OPRAH WINFREY: So tell me, do you have hope now?

CLYDE JACKSON: Yes.

OPRAH WINFREY: There are many who would say, "Why does an inmate deserve hope?" Because they are here because of a crime that they committed, and inevitably took some form of hope away from somebody.

SCOTT KERNAN: Over 90 percent of these inmates will complete their sentence and they'll come back out into the communities. Do you want somebody with no hope, that's involved themselves in criminal activities, doing dope, stabbing people, or would you want a guy that comes out that has an AA degree? Has addressed a substance abuse program? Has-- went to domestic violence classes? What would you want as a taxpayer and a citizen of this State?

OPRAH WINFREY: How has your own personal perception of what it means to be an inmate, a prisoner, how has that changed?

SCOTT KERNAN: When I first came in, that person was the enemy. Now, 35 years later, I don't view the inmates as my enemy. They're people. They're all coming out to be our neighbors. Why wouldn't we spend the resources and create an environment where th-- when they come out, they're better people than when they got here? I just think it makes all the sense in the world. It's common sense.

Clyde Jackson may become part of that 90 percent of inmates to eventually be released. He has a date with the parole board in 2022. He will be 59 years old, and will have been behind bars for more than 40 years.

Aron Franklin, the inmate we met through the bars of the Pelican Bay SHU at the beginning of our story, was returned to the general prison population late last year. But then he attacked a corrections officer - and is now back in solitary confinement.

Produced by Rome Hartman. Sara Kuzmarov, associate producer.