I want to answer his question—of course my experience was different from those of the men at California's Pelican Bay State Prison [13]—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[14] 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 [15]. The only one I'm permitted to speak to is the same person the New York Times was allowed to interview [16] 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 [17].

Like many of the others, Pennington has never been charged with any serious prison offenses, like fighting or selling drugs. In 20 years of incarceration, his only strikes have been two rule violations: delaying roll call and refusing to be housed in a dorm-style cell with at least seven other prisoners. While in prison, he became a certified welder, receiving a special commendation for his work on building a rollover crash simulator for the California Highway Patrol. He used to regularly attend religious services and self-help groups, including parenting classes, Alcoholics Anonymous, and Narcotics Anonymous, all of which are forbidden in the SHU.

Pennington's lawyer, Charles Carbone [18], says his "impeccable prison record" should have him on track for parole. But there is no chance of that—four years ago Pennington was "validated" by prison staff as an associate of a prison gang (one formed on the inside, as opposed to a street gang). That's the reason he and thousands of others are in the SHU with no exit date.

Pennington is not accused of giving or carrying out orders on behalf of any gang. In fact, there is no evidence that he's ever communicated with a member of a gang in his entire life. "I've never been, never want to be a part of no gang," he wrote me. (He is currently trying to challenge his validation in court.)

To validate an inmate as a gang member, the state requires at least three pieces of evidence, which must be "indicative of actual membership" or association with a prison gang in the last six years [19]. At least one item must show a "direct link," like a note or other communication, to a validated gang member or associate. Once the prison's gang investigator has gathered this evidence, it is reviewed in an administrative hearing and then sent to CDCR headquarters in Sacramento.

In Pennington's file [20], the "direct link [21]" is his possession [22] of an article published in the San Francisco Bay View [23], an African American newspaper with a circulation of around 15,000. The paper is approved for distribution in California prisons, and Pennington's right to receive it is protected under state law [24]. In the op-ed style article [25] he had in his cell, titled "Guards confiscate 'revolutionary' materials at Pelican Bay," a validated member of the Black Guerilla Family [26] prison gang complains about the seizure of literature and pictures from his cell and accuses the prison of pursuing "racist policy." In Pennington's validation documents, the gang investigator contends that, by naming the confiscated materials, the author "communicates to associates of the BGF…as to which material needs to be studied." No one alleges that Pennington ever attempted to contact the author. It is enough that he possessed the article.

The second piece of evidence was a cup Pennington had in his cell [27] bearing a picture of a dragon [28], an image CDCR considers an "identifying symbol [29]" of the Black Guerilla Family. The third was a notebook he kept [30], which the gang investigator alleges "shows his beliefs in the ideals of the BGF [31]." Its pages are filled with references to black history—Nat Turner [32], the Scottsboro 9 [33], the number of blacks executed between 1930 and 1969, and quotes from figures like W.E.B. Du Bois and Malcolm X. There are also passages in which Pennington ruminates at length on what he calls "the oppression and violence inflicted upon us here in maximum security," referencing a Time exposé.