In the centre of the Venezuelan capital Caracas, an extraordinary space-age building towers over the sprawling slums below.
El Helicoide was once the symbol of a rich and promising nation.
Today, it houses one of Venezuela's most infamous jails, and embodies the country's decline from Latin American powerhouse to crisis zone.
El Helicoide was built in the 1950s, when Venezuela had big dreams - and big money from oil revenues to match.
The post-World War Two boom was in full swing and dictator Marcos Perez Jimenez wanted to project an image of a forward-looking nation.
"There was real investment in this idea of instant modernity," says Dr Lisa Blackmore, co-author of "Downward Spiral: El Helicoide's Descent from Mall to Prison" and director of Latin American Studies at the University of Essex in the UK.
"[Venezuela] is a country that from 1948 entered a period of military dictatorship, and the mandate was: 'We will progress if we build.'"
El Helicoide was supposed to be world's first drive-thru shopping centre, with ramps spiralling upwards to the 300 planned boutiques in the complex. It was so big it could be seen from anywhere in the city of Caracas.
"This is an absolutely iconic building - there is really nothing like it in Latin America," Dr Blackmore says.
But Perez Jimenez was overthrown in 1958 and this ambitious project became a white elephant.
Place of fear
For years the building stood largely empty, but in the 1980s the government started to transfer some state agencies to El Helicoide - the most important being its intelligence services, now known as SEBIN.
Since then it's become a place of fear, used to detain both ordinary criminals and political prisoners.
The BBC has spoken to former prisoners, their families, legal representatives, NGOs, and also to two former prison guards to build up a picture of life in El Helicoide.
They have asked us to protect their identities because they fear government reprisals against their families.
Rosmit Mantilla arrived at El Helicoide in May 2014. He was one of more than 3,000 people arrested as huge anti-government protests rocked the country.
Aged 32, he was already a well-known political activist and outspoken advocate of LGBT rights.
During his incarceration he would also be elected to the Venezuelan National Assembly, becoming the country's first openly gay congressman.
Economic and political turmoil
Life in Venezuela was becoming increasingly difficult, with spiralling inflation, shortages of food and medicine, and public services nearing collapse.
At El Helicoide, it was a chaotic time. Busloads of prisoners were arriving at the prison every day.
Students, political activists, and sometimes people, including children, were swept up because they'd been in the wrong place at the wrong time.
Mantilla was accused of helping finance the protests - a charge he denies.
Manuel is a former prison officer who remembers Mantilla well.
"[He] was one of those detainees who should never have been there," Manuel remembers.
'Make people afraid'
The former officer told the BBC: "By arresting a lot of people, the aim was to make people afraid."
"And I think they did, in a way. Because nowadays, when there is a demonstration or a march, many Venezuelans are afraid because they don't want to be arrested."
Prisoners in El Helicoide wait days, weeks, even months to appear before a tribunal.
Manuel said: "SEBIN is an institution whose mission is to produce intelligence and information. But for a while that has not been its role. Its role is to defend a regime, a dictatorship."
During his two-and-a-half year nightmare, Mantilla says he was afraid all the time.
But he felt driven to document the torture and cruelty which happened on a daily basis in El Helicoide.
When Mantilla arrived at El Helicoide in 2014, he recalls there were just 50 detainees. Two years later, he says, there were 300.
As the number of detainees rose, the guards had to improvise to find more space.
Offices, toilets, staircases and spaces once meant to be exclusive boutiques were sealed off and converted into cells.
The prisoners had names for them - the Fish Tank, Little Tiger and Little Hell.
But the worst of all was Guantanamo.
"It was an old evidence storeroom," remembers Victor, another former prison officer at El Helicoide. "It was about 12 x 12 metres and it was used to hold around 50 prisoners."
It was hot, cramped and airless.
"No light, no water, no toilet, without sanitation and without beds," says Mantilla. "The walls were stained with blood and excrement."
He told the BBC that prisoners could spend weeks there without showering, urinating in plastic bottles and defecating in plastic bags - which they'd refer to as 'little ships'.
But mistreatment is not the only fear associated with El Helicoide.
All the former prisoners and guards who have spoken to the BBC about their experiences have described the systematic use of torture by the SEBIN to obtain confessions.
Carlos, a former detainee, told the BBC: "They covered my head with a bag. I was badly beaten, kicked and subjected to electric shocks to my head, testicles and stomach."
"I felt immense humiliation, impotence, shame and indignation."
Another detainee, Luis, said: "My head was covered but I heard one SEBIN officer saying: "Let's get the gun. We are going to kill you."
"They were laughing. "There's one bullet. Let's see if you're lucky." I could feel the pistol against my head… and hear them pulling the trigger. It happened many times."
Mantilla says he began to collect prisoners' stories and found the same brutal methods used over and over again.
"There was a university student who had a plastic bag full of human excrement forced over his head so he had to breathe it in," he told the BBC.
"I heard about people raped with blunt objects, others given electric shocks, and some were blindfolded for days until they would lose consciousness."
Human rights abuses
Both former guards who spoke to the BBC denied ever participating in torture themselves, but both say they had seen it happening.
"I saw people being beaten, tied up, suspended from a stair rail by their wrists with their feet barely touching the ground," says Victor.
"They used a battery charger with two wires connected to the detainees through their skin to give them electric shocks," says Manuel.
"[Torture] was systematic," he adds. "It was seen as normal."
Many of these cases have been documented by international human rights organisations, and in February 2018 the International Criminal Court opened a preliminary examination into crimes and human rights violations committed during the protests.
Venezuela has said that it will cooperate with the investigation.
Left to die
In October 2016, after spending two-and-a-half years in El Helicoide, Mantilla became so seriously ill that the prison authorities decided to transfer him to a clinic to undergo surgery.
The procedure was authorised by a judge, but at the last moment the SEBIN intervened. Still in chronic pain, Mantilla was dragged out of the clinic and forcibly returned to El Helicoide, where he was thrown into an isolation cell.
"It was like having a terminal disease and being locked in a room and told you will never be released. They were sentencing me to death."
Video footage posted on the internet of Mantilla shouting defiantly as he was pushed into a SEBIN vehicle outside the clinic caused an outcry both at home and abroad. International humanitarian organisations led calls for his release.
After ten days, the authorities bowed to pressure and Mantilla was transferred first to a military hospital, then to a clinic where he finally had the operation he needed.
He was formally released from detention in November 2016, and within days was sworn in as a congressman - then he began testifying about what he had seen and experienced in El Helicoide.
"Crimes against humanity do not have an expiration date [statute of limitations]," he says.
But Mantilla never felt safe after his release, and in July 2017, he left Venezuela and moved to France. He was granted asylum in May 2018.
From his new home he still follows events in Venezuela closely, and hopes to return one day. His time in El Helicoide continues to cast a long shadow on his life.
"I never was the same person [afterwards]… It is complicated because somehow El Helicoide was my home for two and a half years. Although I try to deny it, a lot of me remains inside there."
Manuel and Victor have also both left Venezuela and are living abroad.
In May 2018, prisoners in El Helicoide rioted in protest at the conditions. A number of detainees were released and promises were made to improve conditions.
But according to accounts from those who have been inside the prison, not much has been done to improve the conditions in El Helicoide.
The BBC repeatedly contacted the Venezuelan authorities and put the allegations about El Helicoide to them.
Despite repeated attempts to contact officials in the Ministry of Communications based in Caracas and to the representatives of the Venezuelan government in the UK the BBC has not received a response to these requests.
Scenes from within the jail are brought to life using vivid 3D illustrations, based on accounts from former detainees and extremely rare interviews with former security officers.
Each chapter gives you a 360-degree scene to explore, by clicking, swiping or using your phone's gyroscope to open up a window into the jail.