You’ve heard it mentioned in countless prison movies—the hole, prison slang for solitary confinement—and it turns out, the truth is pretty ugly.

Let’s define the term first. An inmate in solitary confinement will be housed alone. Generally, the only humans he interacts with will be prion guards, and even that will be brief. He will remain in his cell almost all the time, perhaps being permitted out for a brief period to shower or exercise (also by himself). Solitary has other nicknames too, such as adseg (short for administrative segregation). Solitary is often placed in an area called the SHU, for Secure Housing Unit.

Some of the first US prisons used solitary for all inmates. The theory was that prisoners kept alone would have time to think about their erroneous ways and reform themselves. What happened instead was that, deprived of human contact, the prisoners went crazy. Because solitary confinement was also expensive and restricted inmates from doing most kinds of work, it was soon replaced by other prison models.

But solitary remains in use for several purposes: 1. Punishing prisoners for crimes or misconduct committed while they are locked up; 2. protecting vulnerable prisoners from other inmates; 3. restricting suicidal inmates’ access to objects they could use to harm themselves; and 4. segregating inmates determined to be especially risky, such as gang members.

Conditions in solitary vary a great deal. In some cases, they are very poor. The cell may be extremely small and sometimes devoid of everything except some kind of toilet. The inmate may even be naked. Prisoners in solitary are often given little to do with their time.

There are no limits on the length of time someone can spend in solitary. Some have spent decades there. Many advocates argue that this amounts to torture, especially when the prisoners are young or mentally fragile. It’s also poor policy, in that most prisoners in solitary will eventually be freed but will have a sky-high risk of recidivating.

The US uses solitary to a great extent, and some people claim that this amounts to human rights violations, especially when the confinement is long-term.