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Tuol Sleng, Cambodia - Initially a high school, the building was then transformed into the S-21 Trap and Interrogation Unit during the Pol Pot regime.

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Tuol Sleng, Cambodia - More than 17,000 people were arrested, tortured, and subjected to poor conditions in this security prison. Not many prisoners were able to get out of this prison alive. The site was the stage for a genocide.

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Tuol Sleng, Cambodia - The site is now known as the Tuol Sleng Genocide Museum. Here you can learn more about the victims of the prison and visit the cells and torture rooms.

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Ohio State Reformatory, United States - For more than 100 years, this prison was considered one of the most violent in the US and several people died here.

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Ohio State Reformatory, United States - First used as a juvenile reformatory in 1876, the building later became a prison, capable of housing more than 500 inmates.

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Ohio State Reformatory, United States - The building is now a museum and a tourist attraction. The Ohio State Reformatory also served as inspiration for several movies, including 'Tango & Cash' (1989), 'Air Force One' (1997), and 'The Shawshank Redemption' 1994).

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Eastern State Penitentiary, United States - This building, located in Philadelphia, Pennsylvania, served as a prison between 1829 and 1971.

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Eastern State Penitentiary, United States - The prison was designed to isolate inmates, so that they could get closer to God and get rehabilitated. The prisoners lived in complete isolation, with only a Bible as their possession, and performed tasks such as making footwear and weaving.

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Eastern State Penitentiary, United States - Many historical figures were prisoners here, including the famous mobster Al Capone. Today, Eastern State Penitentiary is a historical site visited by tourists from all over the world.

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Castle of San Juan de Ulúa, Mexico - San Juan de Ulúa is the name of a small Mexican island that houses a complex of fortresses, including a prison.

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Castle of San Juan de Ulúa, Mexico - The castle was designed by the Spaniard Hernán Cortés in 1519. It was used as a prison during the Inquisition and the Porfiriato period, and it was one of the most feared prisons in Mexico.

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Castle of San Juan de Ulúa, Mexico - Prisoners were subjected to poor conditions and even torture. Today the prison is a tourist attraction and a historical symbol of Mexico.

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Fremantle Prison, Australia - This imposing monument was built during the 1850s. At one point, it was used to house prisoners transported from England and Ireland.

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Fremantle Prison, Australia - Many British inmates helped build the prison, which would later become their own home.

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Fremantle Prison, Australia - The prison is listed as a UNESCO World Heritage Site. You can visit it and get to know more about the history of the prison.

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Château d'If, France - Alexandre Dumas's 'The Count of Monte Cristo,' published in 1844, made the prison world-famous as it tells the story of when Edmond Dantès and his mentor, Abbé Faria, were prisoners at the Château.

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Château d'If, France - The French fortification was built between 1527 and 1529 on a small island of the Frioul archipelago in Marseille. The site served as a prison for several years, mainly to house political prisoners.

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Tower of London, England - The site houses the Crown Jewels and was used as a prison for several centuries.

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Tower of London, England - Some say the site is haunted as numerous prisoners were tortured and executed onsite, including Queen Anne Boleyn and Lady Jane Gray.

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Alcatraz, United States - Alcatraz Federal Penitentiary was a federal maximum security prison, located on Alcatraz Island, off the coast of San Francisco. The prison was in operation from 1934 to 1963.

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Alcatraz, United States - It was designed to house prisoners who were constantly causing problems in other federal prisons and became one of the most famous and feared prisons in the US.

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Alcatraz, United States - Several prominent criminals, including Al Capone, Robert Franklin Stroud, Bumpy Johnson, Rafael Cancel Miranda, Mickey Cohen, and James 'Whitey' Bulger, were prisoners in Alcatraz.

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Alcatraz, United States - The iconic prison is today a tourist hot spot and has been used as a set in many movies.

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Kilmainham Gaol, Ireland - Kilmainham Gaol was a prison located in Kilmainham, Dublin. It is now a museum.

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Kilmainham Gaol, Ireland - The prison opened in 1796 and many of the leaders of the Irish rebellion were imprisoned and executed here.

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Kilmainham Gaol, Ireland - In addition, numerous other prisoners were executed and tortured in this prison.

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Robben Island, South Africa - This prison located in Cape Town drew attention primarily during apartheid as it housed the man who would be the country's first democratically elected president, Nelson Mandela.

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Robben Island, South Africa - Before that, the island was used as a leper colony between 1836 and 1931.

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Robben Island, South Africa - The prison is now a UNESCO World Heritage Site and also one of the most emblematic monuments in South Africa.

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Port Arthur, Australia - This prison, located in the state of Tasmania, was used specifically to house the most dangerous criminals and repeat offenders.

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Port Arthur, Australia -

Today it's a UNESCO World Heritage Site and a popular tourist attraction in Tasmania.

See also: The world's most dangerous prisons

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