BOLIVIA: San Pedro Prison

It is 20 years since Bolivia's chaotic San Pedro Prison became a tourist attraction, a place where backpackers stayed for weeks and partied with inmates.

The story was immortalised in a best-selling book, Marching Powder, and now the book's protagonist - a former inmate - and author have reunited to return to the prison for a new documentary film.

The inmate, Thomas McFadden, says returning to his old cell scared him. "But I always wanted to go back to Bolivia, and there was always a strong connection between me and Rusty."

Rusty Young wrote Marching Powder in his twenties, when as a young backpacker from Sydney, Australia, he stayed in the prison, and later helped secure Mr McFadden's release by posing as an international human rights lawyer.

It all began when Mr McFadden, a British-Tanzanian, was caught smuggling cocaine in La Paz airport in 1996 and found himself jailed in a place more like a warped small town than a prison.

Prisoners were expected to earn a living inside and buy their cells as if they were real estate. There was a primary school for the prisoners' children, who they brought to live with them, and in the most notorious twist of all, there was an in-house cocaine factory.

"I thought what is this place? I even had to pay my own taxi fare to get there," Thomas McFadden remembers today.

After his release in 2000, he left those days far behind and is now a chicken farmer in Tanzania. However, his life-changing friendship with his "brother Rusty" has gone the distance, and he has even named one of his children after him.

Their unusual relationship began when Mr Young joined one of the illegal prison tours that the inmate ran for foreign backpackers from early 1997 until his release.

Word had spread around local hostels that there was an English-speaking inmate who would show visitors around this strange microcosm.

Centrally located in downtown La Paz, the prison became a tourist attraction that featured in Lonely Planet guidebooks. Travellers paid an entrance fee and, in some cases, opted to stay for weeks, partying with the inmates.

Lydia Docking, a British traveller, went on not just one but repeated trips to the prison in 2008, led by a prisoner from Portugal. One of the other backpackers she met on her first day there later became her husband.

"As a conversation starter, we used to say we met in a Bolivian jail," she says from her new home in Sydney. She was 24 at the time and is 33 now.

Their San Pedro tour guide came to their wedding after his release, and although the couple has since separated, she still has their dog, named Pedro after the prison.

Despite her jokes, she is acutely well aware of the darker side of the story. "I'm still confused by it, still processing it," she says. "Over the course of three weeks, we went most days. I sometimes question why. It was mostly just to share experiences and to understand how lives start the same but can take such different paths."

San Pedro Prison remains operational today, despite many plans to close it. For years, its tours were an open secret, but one day in 2009 a local television crew arrived to get footage for a story about a politician held inside. While waiting in the plaza outside, the cameramen caught a steady stream of tourists going in.

Armed with the footage, the Bolivian press ran with the story. The main concern locally was not the tourists but the alleged corruption of the authorities, who were thought to be taking a cut of the tours' profits. "Who is watching the police?" asked an editorial in La Razon newspaper. During the ensuing "clean-up", prisoners had their visitors' rights revoked for a day. A riot followed and around 80 children were evacuated.

In 2013, the story took an even darker turn when news spread of a 12-year-old girl becoming pregnant inside the jail, after an alleged rape by her imprisoned father and other men. Child welfare charities called for a ban on children living inside.

Large-scale tours of the prison are no longer possible. Today backpackers typically opt for La Paz walking tours, which stop outside the prison walls and tell Marching Powder's story.

When Rusty Young and Thomas McFadden returned to the prison for their forthcoming documentary Wildlands, they happened upon one such tour group and snuck alongside them, incognito.

"They kept making factual errors, so I couldn't help asking smart-arsed questions," says Mr Young. Eventually an Irish traveller, who had just read the book, recognised the pair. "He whispered, 'You're Rusty, You're Thomas!' And we sneaked off together for a beer."

Back in 2000, the two men travelled to Colombia, after Mr McFadden's release. They found work as English teachers and worked on the book, which became a New York Times best-seller.

Thomas then moved to Tanzania, and Rusty went back to Australia, where after the huge success of Marching Powder he had difficult-second-book syndrome. His novel Colombiano, about a teenager who gets drawn into a Colombian terrorist organisation, is due out later this year.

The documentary Wildlands uses their story in San Pedro Prison as a launch pad for exploring all sides of the drugs trade, and includes interviews with former undercover US police agents and an assassin who was a right-hand man to Colombian drug lord Pablo Escobar.

But the long-awaited Marching Powder feature film may still be years away. The rights were acquired by Brad Pitt's production company but have since expired. Narcos director Jose Padilha has been linked to the screenplay, and Don Cheadle and then Chiwetel Ejiofor expressed interest in playing Thomas McFadden, but filming never got off the ground.

As for the sequel to the book, perhaps that will come sooner. "My publishers been harassing me about that for years," says Rusty. "We already have a title: Marching On."