This huge abandoned fortress hidden inside a mountain holds a world of subterranean secrets.

A Stalinist prison camp for the wives and children of men accused of betraying the Soviet state is now a memorial museum.

The wartime prison now features haunting sculpted reenactments of the atrocities that took place there.

Located at the former Lorton Prison the Lucy Burns Museum tells the story of the 91 year history of the prison including the dark chapter of its involvement in the women's suffrage movement.

A former prison complex now displays a historic collection of inmate artwork.

An abandoned Ecuadorian prison that once held ex-presidents and notorious con artists is now a carceral museum.

On the side of Arkansas' Great River Road stand the remains of a forgotten World War II POW Camp.

The antique hotel still has a jail in the basement, though the cockfighting ring is long gone.

The now-abandoned Fascist summer colony served a dark purpose during World War II.

The watery entrance for condemned prisoners heading to the Tower of London is still visible along the Thames.

Haunting graffiti left by prisoners of the Spanish Inquisition.

This maximum-security prison was abandoned to make way for a national park.

This almost-forgotten underground dungeon housed several convicts over the years, including Caravaggio.

The maximum security facility was once thought to have "the bloodiest 47 acres in America."

This oddly placed 15th-century church is one of only six bridge chapels left in England.

This jail once controlled by the Japanese military now sits abandoned on a remote Pacific island.

The notorious "Hanoi Hilton" where Vietnamese colonial subjects and later American POWs, including John McCain, were imprisoned.