IT’S BEEN SEVEN MONTHS since I’ve been inside a prison cell. Now I’m back, sort of. The experience is eerily like my dreams, where I am a prisoner in another man’s cell. Like the cell I go back to in my sleep, this one is built for solitary confinement. I’m taking intermittent, heaving breaths, like I can’t get enough air. This still happens to me from time to time, especially in tight spaces. At a little over 11 by 7 feet, this cell is smaller than any I’ve ever inhabited. You can’t pace in it.

Like in my dreams, I case the space for the means of staying sane. Is there a TV to watch, a book to read, a round object to toss? The pathetic artifacts of this inmate’s life remind me of objects that were once everything to me: a stack of books, a handmade chessboard, a few scattered pieces of artwork taped to the concrete, a family photo, large manila envelopes full of letters. I know that these things are his world.

“So when you’re in Iran and in solitary confinement,” asks my guide, Lieutenant Chris Acosta, “was it different?” His tone makes clear that he believes an Iranian prison to be a bad place.

He’s right about that. After being apprehended on the Iran-Iraq border, Sarah Shourd, Josh Fattal, and I were held in Evin Prison’s isolation ward for political prisoners. Sarah remained there for 13 months, Josh and I for 26 months. We were held incommunicado. We never knew when, or if, we would get out. We didn’t go to trial for two years. When we did we had no way to speak to a lawyer and no means of contesting the charges against us, which included espionage. The alleged evidence the court held was “confidential.”

What I want to tell Acosta is that no part of my experience—not the uncertainty of when I would be free again, not the tortured screams of other prisoners—was worse than the four months I spent in solitary confinement. What would he say if I told him I needed human contact so badly that I woke every morning hoping to be interrogated? Would he believe that I once yearned to be sat down in a padded, soundproof room, blindfolded, and questioned, just so I could talk to somebody?

When, after five weeks, my knees buckled and I fell to the ground utterly broken, sobbing and rocking to the beat of my heart, it was the patch of sunlight that brought me back.

I want to answer his question—of course my experience was different from those of the men at California’s Pelican Bay State Prison—but I’m not sure how to do it. How do you compare, when the difference between one person’s stability and another’s insanity is found in tiny details? Do I point out that I had a mattress, and they have thin pieces of foam; that the concrete open-air cell I exercised in was twice the size of the “dog run” at Pelican Bay, which is about 16 by 25 feet; that I got 15 minutes of phone calls in 26 months, and they get none; that I couldn’t write letters, but they can; that we could only talk to nearby prisoners in secret, but they can shout to each other without being punished; that unlike where I was imprisoned, whoever lives here has to shit at the front of his cell, in view of the guards?

“There was a window,” I say. I don’t quite know how to tell him what I mean by that answer. “Just having that light come in, seeing the light move across the cell, seeing what time of day it was—” Without those windows, I wouldn’t have had the sound of ravens, the rare breezes, or the drops of rain that I let wash over my face some nights. My world would have been utterly restricted to my concrete box, to watching the miniature ocean waves I made by sloshing water back and forth in a bottle; to marveling at ants; to calculating the mean, median, and mode of the tick marks on the wall; to talking to myself without realizing it. For hours, days, I fixated on the patch of sunlight cast against my wall through those barred and grated windows. When, after five weeks, my knees buckled and I fell to the ground utterly broken, sobbing and rocking to the beat of my heart, it was the patch of sunlight that brought me back. Its slow creeping against the wall reminded me that the world did in fact turn and that time was something other than the stagnant pool my life was draining into.

Here, there are no windows.

Acosta, Pelican Bay’s public information officer, is giving me a tour of the Security Housing Unit. Inmates deemed a threat to the security of any of California’s 33 prisons are shipped to one of the state’s five SHUs (pronounced “shoes”), which hold nearly 4,000 people in long-term isolation. In the Pelican Bay SHU, 94 percent of prisoners are celled alone; overcrowding has forced the prison to double up the rest. Statewide, about 32 percent of SHU cells—hardly large enough for one person—are crammed with two inmates.

The cell I am standing in is one of eight in a “pod,” a large concrete room with cells along one side and only one exit, which leads to the guards’ control room. A guard watches over us, rifle in hand, through a set of bars in the wall. He can easily shoot into any one of six pods around him. He communicates with prisoners through speakers and opens their steel grated cell doors via remote. That is how they are let out to the dog run, where they exercise for an hour a day, alone. They don’t leave the cell to eat. If they ever leave the pod, they have to strip naked, pass their hands through a food slot to be handcuffed, then wait for the door to open and be bellycuffed.

I’ve been corresponding with at least 20 inmates in SHUs around California as part of an investigation into why and how people end up here. While at Pelican Bay, I’m not allowed to see or speak to any of them. Since 1996, California law has given prison authorities full control of which inmates journalists can interview. The only one I’m permitted to speak to is the same person the New York Times was allowed to interview months before. He is getting out of the SHU because he informed on other prisoners. In fact, this SHU pod—the only one I am allowed to see—is populated entirely by prison informants. I ask repeatedly why I’m not allowed to visit another pod or speak to other SHU inmates. Eventually, Acosta snaps: “You’re just not.”

IF I COULD, I WOULD MEET WITH Dietrich Pennington, a 59-year-old Army veteran from Oakland who has served 20 years of a life sentence for robbery, kidnapping, and attempted murder. Pennington has lived alone in one of these cells for more than four years. During that time, he hasn’t spoken to his family. He has never met any of his seven grandchildren. In the SHU, he’s seen “some of the strongest men I know fall apart.”

But the fact that Pennington is in solitary is not what is remarkable about his story. More than 80,000 people were in solitary confinement in the United States in 2005, the last time the federal government released such data. In California alone, at least 11,730 people are housed in some form of isolation. What is unique about Pennington—if being one of thousands can be considered unique—is that he doesn’t know when, or if, he will get out of the SHU. Like at least 3,808 others in California, he is serving an indeterminate sentence.

Compared to most SHU inmates, Pennington is a newbie. Prisoners spend an average of 7.5 years in the Pelican Bay SHU, the only one for which the California Department of Corrections and Rehabilitation (CDCR) has statistics. More than half of the 1,126 prisoners here have been in isolation for at least five years. Eighty-nine have been there for at least 20 years. One has been in solitary for 42 years.