26 children boarded a school bus. 11 hours later, they were all buried underground.

Twenty-six excited children packed themselves onto a yellow school bus in California. They’d just won a petition to extend their summer school by two weeks and they were beyond elated.

Because, now, what should have been their second-to-last day had turned into their twelfth-to-last, meaning 12 more sun-soaked days of swimming, arts and crafts, and park games.

But that same afternoon of July 15 in 1976, the lives of these 26 children – aged 5-14 – were about to turn upside down, putting them at the centre of the biggest mass kidnapping in US history.

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As the students, from Dairyland Elementary School in Chowchilla, travelled home from the local pool along a narrow rural road, the bus driver Edward Ray noticed a van plonked right in the middle. It had its hood up and it was blocking the route.

Ray had no choice but to slow down and try to manoeuvre his clunky bus around the stationary vehicle.

Just as Ray leaned out to offer to help the owners of the stranded van, three masked men surged onto the school bus. Machine guns were pointed towards the children’s faces. Some were still in their swimming suits.

The gunmen took the wheel and began speeding it through a tall bamboo thicket, the stalks knocking the bus – and the students inside – from side to side.

The school bus eventually reached a hidden ditch where two vans were stationed. The children and their bus driver were ordered to get inside.

The gunmen then took the children on an 11-hour drive. They weren’t told where they were going, and all the windows were blacked out. They were sweltering in stuffy, 38 degree heat. They had no water, no food and no toilet, forcing frightened children to soil themselves.

Only when night fell did the vans finally come to a stop. The 27 hostages found themselves in a rock and gravel quarry near Livermore – only about two hours from Chowchilla.

“I’m sure they had to (wait) until a time when they knew no one would be able to be around, no workers, to see 26 children get buried,” recalled Lynda Carrejo Labendeira, who was a fourth-grader at the time, to CNN.

The gunmen unloaded the their young victims and demanded their names, ages, addresses and phone numbers.

The children and bus driver were then shoved one-by-one down a ladder into what they thought was a mass grave, made from a moving van buried deep underground. They were locked inside.

The next 16 hours were a nightmare. Some of the children became convinced they were going to die.

"It was buried into the earth. It was like a tomb... It was like a giant coffin for all of us," Carrejo Labendeira said.

The conditions inside the makeshift cell were wretched. The kidnappers left a paltry amount of water and only enough cereal, peanut butter and bread for one meal.

Next to the food, a box was carved into a toilet. Dirty mattresses were strewn along the floor, and there was no ventilation.

Before long, the underground prison began to reek of vomit, urine and excrement. Screams and cries echoed inside the van.

But Ray refused to give up hope.

Ray described the ordeal to ABC in 1976: "The ceiling started to cave in and everything else. We thought we were going to have it right then, but kept begging to let us out."

He hatched a plan with some of the older students that involved stacking the mattresses and escaping through a metal lid above.

When they clambered to the top, they realised the plate was weighted down with two 45kg industrial batteries and covered by metres of dirt. After a lengthy struggle, they finally managed to wedge the lid open and dig through the dirt that was blocking them from the outside.

It was an extraordinary feat. Children clambered through the escape path, some climbing on top of each other's shoulders.

And after 16 exhausting, harrowing hours, they were free. With the gunmen asleep, the hostages ran for help and found a small building nearby. The man inside knew who they were before they uttered a word.

"The gentleman came down and said, 'This world's been looking for you.' He knew exactly who we were," Carrejo Labendeira said.

At last, the children were reunited with their panicked parents, 36 hours after their school bus was hijacked.

Perhaps most puzzling about the mass kidnapping was the lack of a solid motive for the ordeal. Detectives thought they were inspired by Hollywood.

The three gunmen - Richard Schoenfeld, his brother John Schoenfeld and their friend Fred Woods - were young men from wealthy families in Northern California. (Even the quarry the van was buried in was owned by Woods' father.)

The three criminals spent 18 months crafting their plan. They wanted a $5 million ransom. Only, the ploy failed from the start. Their ransom call never reached the police because their phone lines were so overloaded.

The kidnappers were quickly arrested and sentenced to life in prison.

Richard was paroled in 2012 and his brother James walked out of prison a free man just a few years later in 2015.

In October 2019, Woods was denied parole for the 17th time, according to information provided to CNN by the California Department of Corrections and Rehabilitation. He will be eligible for the 18th attempt in 2024.

But for the young children who 40 years later have grown into adults, the trauma never left. Because while they were never physically injured, the psychological impacts of being snatched at gunpoint and buried alive were enormous. They developed various mental health issues, including depression, anxiety, addiction, schizophrenia and claustrophobia.

"I still sleep with a night light," said Jennifer Brown Hyde, who suffers from chronic nightmares, to People magazine. "[I] can't ride a subway or go underground."

Another victim, Larry Park, said: "'After the kidnapping I started to hear a voice in my head. When I was 11, it turned violent. I fantasised about killing my kidnappers."

Darla Neal told CNN her "extreme anxiety" made it impossible to live normally.

"I'm overwhelmed to the point that I had to leave work. I tell myself I should be able to shake this off and deal with it. Yet here I am, a mess."

In 2016, a group of victims joined together to file a lawsuit for the damage they suffered.

Still, the 26 former schoolchildren will forever hold the memory of the bus driver close to their hearts. Ray received a community service award for his heroism, and was visited many times by the students he saved.

When he died in 2012, one of the survivors wrote on the bus, which is now housed in a museum: "You will forever be my hero."