Fort of San Cristóbal, Spain

A huge underground fortress was carved into a mountain in the early 1900s. In Spain’s Civil War, Franco made the fortress into a prison, where thousands of men were incarcerated within its maze of tunnels and corridors. The prison was notorious for its harsh conditions and many prisoners died of starvation, tuberculosis and executions and were buried secretly in mass graves. These days graffiti writers and paranormal investigators explore its subterranean depths.