Imagine a place filled with closed, windowless cells. Each cell may be so small that you can extend your arms and touch the side walls. It may contain a bunk of poured concrete, a toilet, perhaps a small table and stool. A few personal possessions – books, family photos – may be permitted, or they may not. The door to the cell is solid steel.

Three times a day, a food tray slides in through a slot in the door; when that happens you may briefly see a hand, or exchange a few words with a guard. It is your only human contact for the day. Five times a week, you are allowed an hour of solitary exercise in a concrete-walled yard about the same size as your cell. The yard is empty, but if you look straight up, you can catch a glimpse of sky.

Imagine that a quarter of the people who live in this place are mentally ill. Some have entered the cells with underlying psychiatric disabilities, while others have been driven mad by the confinement and isolation. Some of them scream in desperation all day and night. Others cut themselves, or smear their cells with faeces. A number manage to commit suicide in their cells.

You may remain in this place for months, years, or even decades. The conditions in which you live have been denounced as torture by UN officials and by a host of human rights, civil liberties, and religious groups. And yet you remain where you are.

This place is located not in some distant authoritarian nation or secret black site abroad, but here on US soil. In fact, there are places like it in nearly every state in the union, within sight of our own cities and towns. On any given day in the United States, supermax prison and solitary confinement units hold at least 80,000 men, women, and children in conditions of extreme isolation and sensory deprivation.

Most of them have committed nonviolent offenses against prison rules or have been categorically branded as "high risk". A large and disproportionate percentage suffer from serious mental illness. Some of them are children. Condemned to solitary by prison officials, they spend 23 hours a day in their cells without work, rehabilitative programming, or human contact of any kind.

These prisoners live out of sight of the public and the press. Their conditions have, with few exceptions, been condoned by the courts and ignored by elected officials. As a result, over the past three decades, the use and abuse of solitary confinement in US prisons has grown into one of the nation's most pressing domestic human rights issues – yet it also remains one of the most invisible.

On Tuesday, for the first time, the US Congress has taken a look at these domestic black sites. The Senate judiciary subcommittee on the constitution, civil rights, and human rights held a hearing in which corrections officials, lawyers, and mental health experts – along with one lone survivor of prison isolation – testified to the "human rights, fiscal, and public safety consequences" of solitary confinement.

For evidence of humanitarian consequences, the senators need only turn to their colleague John McCain, who spent two years in solitary confinement as a prisoner of war in Vietnam (in a cell somewhat larger than those in most American supermaxes). "It's an awful thing, solitary," McCain later wrote. "It crushes your spirit and weakens your resistance more effectively than any other form of mistreatment."

As for fiscal and public safety consequences, the subcommittee members can look to evidence-based research that keeping prisoners in solitary confinement costs two to three times more than keeping them in the general population, and is likely to increase both prison violence and recidivism. Or they can study the example of the few states – including Maine and Mississippi – that dramatically reduced the number of prisoners they keep in isolation, with positive results.

What remains to be seen is whether Congress will take further action to curb this failed and torturous practice. Given the political will, the subcommittee could begin by holding more hearings around the country, while its staff carries out an investigation that opens up to public scrutiny the tormented inner workings of supermax prisons and solitary confinement units.

An independent federal body with the absolute right to enter and report on prisons could go even further in exposing abusive conditions. Legislation could then force the creation and adoption of federal standards for the treatment of prisoners, which states would have to meet in order to receive federal funds.

All of this depends upon our elected leaders taking seriously the notion that all Americans – including prisoners – have an absolute right to immunity from torture by the state. That is likely to happen any time soon, but until it does, unimaginable things will keep taking place at black sites in our own backyards.