Prison Diaries: Inmates Describe Living in Solitary Confinement Three prisoners at the New Mexico State Penitentiary share their stories.

June 19, 2014 -- intro: An estimated 80,000 inmates are locked up in solitary confinement in the United States, but this form of incarceration has become extremely controversial. Not only can it be three times as expensive to house an inmate in solitary, but critics say it can also make inmates more dangerous.

“Nightline” followed Gregg Marcantel, the secretary of corrections for the state of New Mexico, as he spent 48 hours undercover in solitary confinement at the New Mexico State Penitentiary, in Santa Fe. Marcantel wanted to experience solitary himself to help him decide how to reform the state's use of this practice.

Along the way, “Nightline” spoke to inmates who were sent to solitary confinement because of their violent past and gang affiliation, some of whom had spent years inside a cell alone that is smaller than an average parking space.

Here are some of their stories:

quicklist: title: Freddy Munoz media:24204998 text:Growing up, Freddy Munoz, 34, said he wanted to be an astronaut, but at age 13 he got caught up in gangs and committed two murders. He has been in a solitary cell for years because of his violent past and gang affiliations. He knows every crack and every inch of peeling paint in his cell.

“It is perpetual misery,” Munoz said. “It's ennui. It's monotony. It's repetition.”

quicklist: title: Daniel Herrera media:24205039 text: Daniel Herrera was also placed in solitary because he was a gang member. He is 23 years old and serving a sentence for kidnapping.

Herrera gave a tour of his cell, showing his bed, reading materials, including a Bible and a state-issued television.

quicklist: title: Nathaniel Stein media:24217380 text:Solitary confinement has been called legalized torture, but many corrections officials say it’s a necessary tool to control a dangerous prison population. Officers at the prison point to inmate Nathaniel Stein as an example. Stein, who is in prison for robbery and burglary, was placed in solitary after he viciously attacked a corrections officer a few months ago.