Tonight, there are an estimated 80,000 people locked up in solitary confinement in America. You're about to meet a prison official who wants to experience it firsthand. He's embarking on a radical,... See More
Tonight, there are an estimated 80,000 people locked up in solitary confinement in America. You're about to meet a prison official who wants to experience it firsthand. He's embarking on a radical, risky experiment, entering level six solitary confinement himself, undercover. Dan Harris brings us this report "No way out." Reporter: You are looking at prisoner 106, on the books as a dangerous, armed robber. He is prepares to enter what many call hell on Earth. Solitary confinement. Before you meet its residents, just listen to them. This is prison. You know, we're not here for being nice. Moved into the unit -- I don't know that I have ever felt as vulnerable as I do right now. Reporter: The thing almost nobody here knows is that this inmate is actually the secretary of corrections for the state of New Mexico, a former cop who spent a career putting away bad guys, he's now going to try to spend 48 hours living among the worst of the worst. This surveillance footage shows what these men are capable of if they get loose. This is a public safety issue. Reporter: So why is he willing to put himself through it? He decided he wants to reform the state's use of solitary, because most of these inmates will end up back on our streets. We're sending people back to our neighborhoods worse than when they came. Reporter: As the clock starts running on his 48-hour lockdown, he settles into his 12 X 7 concrete cell. Level six is divided into three separate units. Housing a total of 282 inmates. In here for crimes raging from rape to armed robbery to murder. Combined, these men have killed 138 people. In cell 111 is Freddie Munoz. This is my home right here, for good or bad, this is it. Reporter: Growing up, he wanted to be an astronaut. But at age 13, he got caught up in gangs. How many murders did you commit? Two. Reporter: Because of his violent past and gang affiliation, officials have kept him in a cell like this every day for the past ten years. These four walls are all he's got. He knows every crack, every inch of peeling paint. What does that do to your head? It's perpetual misery. It's monotony, it's repetition. Reporter: He says the only way to keep it together in here is to establish a strict routine, a sort of imitation of life. I exercise. I read a lot. If I didn't have books, I probably would have gone insane. Reporter: A few cells away, Daniel Herrera. He's also in solitary because of his gang affiliation. He's 23 years old and serving a sent tense for kidnapping. This is where we live. Right here you can see the width of our room. Reporter: At our request, he shows us one of the tricks of solitary. This is called fishing. A fishing line, to communicate with neighbors and stuff like that. You send it to your neighbor. That's how we fish and get things from one side to another in here without getting caught or getting in trouble. Reporter: Back in the secretary's cell, he brushes his teeth and gets ready for a long night in a strange and hidden world. The officer makes his morning rounds. It's chow time. ! Reporter: When collecting the food trays, he makes sure nobody has broken off a piece of plastic, which could be used to create a piece of weapon. Is this a dangerous job? It takes a certain type of person to do it. We're inside the lion's cage with the lion. Reporter: To keep safe, they follow rigid protocols. There's a strip search for weapons every time they take an inmate out to the yard, known as the rec cage. ? Locked down 23 hours a day, reminiscing how my life was stripped away ? Reporter: And coming back is equally as coordinated. Inmates get an hour a day outside their cell. Being walked back to your cell by two guards is probably the only human contact you get all day. It's 24 hours into a stay, and the confinement is starting to get to the secretary. He's trying to pass the time by working out, but there's only so much of that you can do. Claustrophobia is setting in. I fell like the cell was squeezing down on me. Reporter: He's getting a taste of why this form of punishment has become so controversial. Not only can it be three times as expensive as regular incarceration, but critics call it legalized torture. We are social animals, built for interaction. Take that away and critics say inmates can literally lose their minds. But many corrections officials stay it's a necessary tool to control a dangerous population. As a prime example, they point to an inmate like nethanial. Would you consider yourself a dangerous man? No. Reporter: However, he's in solitary for viciously attacking a corrections officer. Why did you do that? Tough question. It's just the situation. I don't know why. Reporter: I notice when you are fessing up to something, you do smile. Yoi don't know why I do that. Reporter: The signal to me is you don't feel bad about it. I don't really feel bad for things I've done. Reporter: On the flip side, there are inmates like Freddie who has been a model inmate and renounced his gang affiliation. Munoz is now one of a select few hand picked to move out of solitary and into the general population of the prison, as part of the secretary's plan to reduce the population in solitary. To be able to go outside without being restrained every day, I think that would be very good for me. Reporter: It's the final morning for this undercover boss. Finally, the moment he's been waiting for. Release. After 48 hours on the inside, he's concluded solitary should be used, but sparingly and only for the most dangerous. We have people who say, anymore more than 30 days is torture. Do you agree with that? No, I don't. There is such thing as evil, and it's up to me to make sure I'm protecting other inmates and my staff. Reporter: Days later, several inmates are brought out of level six solitary in preparation for an eventual return to the general prison population. Here, for the first time in years, they'll be able to sit next to one another and have real human contact. It's great. This is living right here, finally. Reporter: These men, part of an experiment and part of a trend as New Mexico and the nation take its first steps away from this controversial form of punishment. For "Nightline," this is Dan Harris, in Santa Fe, New Mexico. This is a controversy you'll hear a lot more about. Thanks, Dan.
This transcript has been automatically generated and may not be 100% accurate.