GEOFF BENNETT: The Assad legacy is one of horror, hundreds of thousands dead, millions displaced or in exile.

But perhaps nothing illustrates the deep depravity more than the atrocious archipelago of prisons and torture centers where tens of thousands were killed by the regime.

AMNA NAWAZ: Special correspondent Leila Molana-Allen takes us inside some of these now-liberated hells on earth, as Syrians search for answers and for their loved ones.

And a warning: This story contains many images and accounts of violence.

LEILA MOLANA-ALLEN: The word that has struck terror into the hearts of every Syrian for decades, Sednaya, a black hole of pain and abuse where tens of thousands disappeared.

Few ever emerged again.

But now those who feared these walls have breached them, desperately searching for survivors of the deposed President Bashar al-Assad's nightmarish Damascus prison.

A sudden swarm.

They have heard a sound.

This group of family members and fighters have found a steel wall, and they think they can hear voices behind it.

They're searching now, trying to break through to see if there may be any prisoners behind it still alive.

Rescue crews have been searching the vast mountain compound for days following rumors of a network of hidden cells underground.

Pounding through layers of concrete, they found nothing.

But with tens of thousands more detainees still missing, these families are holding on to that fading hope, and everyone here is determined to leave no stone unturned.

Outside, more crowds and pleading faces.

Malak's 16-year-old son was taken seven years ago.

She never heard from him again, but the government told her he was alive in jail.

MALAK HILAL, Mother (through translator): What was his crime?

He was 17.

He missed out on his childhood, and I didn't get to see him grow up.

He is the sweetest young man.

LEILA MOLANA-ALLEN: Like so many here, she says her son did nothing to deserve this.

She was never told why he was imprisoned.

When Sednaya was broken open last weekend, she jumped on a bus from her home far in the north.

For three days, she has sat here in the freezing cold with other gathered mothers, waiting.

MALAK HILAL (through translator): I don't want anything except my son.

My heart is telling me that my son is here, like he is calling for me, mama.

LEILA MOLANA-ALLEN: Inside the prison's darkened walls, a trove of documents scattered across every floor.

Each contains precious details that could reveal news of a loved one.

Syrians scour through the crumpled pages by the light of their cell phones, looking desperately for a lead on a loved one.

Hurriedly abandoned documents list the names of women held underground, fingerprints of inmates, and every now and then signs of the children we now know were held here, a coloring book.

Kids as young as 13 were imprisoned here and some were born here, the horrifying legacy of the systematic rape of female inmates; 14-year-old Malik is helping his family search for his cousin.

He hopes he will never have to face what other boys his age have.

MALIK, Cousin (through translator): I hope now the oppression in this country will end and children will be free to go to school and not fear being arrested.

LEILA MOLANA-ALLEN: But hope for everyone here is turning to despair, as the evidence of Assad's torture chambers inside the walls of the prison becomes clear, a wall of death, nooses laid out for public hangings and, then the crusher.

MAN (through translator): They bring a prisoner and put him between these metal plates and crush him between them.

Then they scrape out the flesh and bones and throw it in a bag.

LEILA MOLANA-ALLEN: Human beings treated like waste.

With thousands of inmates crammed within these walls, prisoners say there were daily executions, sometimes just to make space.

In the bowels of the complex, more horrors await.

Many who were kept down here did not see daylight for years.

Prisoners say this hole was for daily humiliation.

The guards filled it with urine and feces and would force them into it.

On the walls, the fevered scratchings of those who tried to keep a sense of time and life outside.

For years, we have searched for evidence of what exactly was happening in this place they called the human slaughterhouse based on snippets of information that came from the few people who managed to make it out alive.

It's utterly haunting now to walk these halls and know that these tiny cells are where so many thousands of human beings endured unimaginable suffering.

The minds of the thousands who survived these torture chambers remain haunted.

MOHAMMAD DALEEL, Former Prisoner (through translator): The Assad regime considered us terrorists.

If you were married, they'd bring your wife and beat her and her friends.

They would torture, rape and humiliate them.

Some stayed in darkness all day.

Some would die from the torture.

We were constantly beaten.

I saw the cruelty for myself, and so many people died around me.

LEILA MOLANA-ALLEN: Mohammad was arrested at 17.

He spent three years in another prison.

He was finally released, only to be immediately conscripted into the army by force.

He defected as soon as he could, but the authorities tracked him down and threw him in Sednaya.

MOHAMMAD DALEEL (through translator): I was so terrified.

If we hadn't been saved by our brothers I would have gone insane.

Those that were with me had already gone insane.

LEILA MOLANA-ALLEN: Daily physical and psychological torture have taken their toll, but unimaginable just a few days ago, Mohammad is home.

Some of the thousands still searching are now forced to accept the horrifying truth, that a funeral shroud may be better than no news at all.

There is a box with bones, just bones.

Dozens of bodies were discovered at a military hospital next to Sednaya yesterday and were brought here to Damascus' Mujtahid public hospital, in various states of decay, many with limbs missing.

Some seem only to have been killed in the last few weeks, their emaciated bodies covered with the scars of torture and starvation, faces stretched in a rictus of pain.

MOHAMMAD TAYSEER: Medical Student: They have been crushed, burnt with acid.

They have been treated like -- sorry to say, but animals were - - animals were better than them in Sednaya.

LEILA MOLANA-ALLEN: All day, families have come to collect the bodies of loved ones they have identified.

MOHAMMAD TAYSEER: If you see there, there is a box that's filled with bones.

Their skins and meat was burnt in acid.

LEILA MOLANA-ALLEN: Room after room of the morgue is laden with corpses.

This man is searching for his son.

A doctor tries to help him find identifying features like teeth and eyebrows.

Sobbing mothers and shell-shocked fathers file through in anguish, barely able to bring themselves to look at the rows of young men, desperate not to see their children among them, but driven by desperation to discover their fates.

A young man rocks in the corner of the morgue, inconsolable.

For those too destroyed to be identified, DNA tests.

They died alone in an unimaginable hell.

These heartbroken families, faint hopes now gone, pray they can show them the respect they deserve in death.

Across Syria, thousands of grieving families may soon no longer be looking for mass prisons, but mass graves.

For the "PBS News Hour," I'm Leila Molana-Allen in Sednaya Prison, Damascus.