An enormous underground fortress hides within a Spanish mountaintop. Officially closed and abandoned by the military, its maze of silent corridors leads urban explorers, graffiti writers, and investigators of the paranormal alike deep into its world of subterranean secrets and horrors.

Originally viewed as a turn-of-the-century fortification in the early 1900s, the San Cristóbal Fortress was soon rendered obsolete with the emergence of airstrike operations. During the Spanish Civil War and Franco’s dictatorship, it was transformed into a prison that hosted thousands of men within its damp walls.

During the fort’s stint as a prison, it became known for its harsh conditions. Summary executions, starvation, and tuberculosis were occurrences within the dank, depressing space. It wasn’t long before the mountain became filled with unnamed corpses, secretly buried in mass graves.

Not all captives were willing to accept their harsh fates. Beneath the light of a full moon in May of 1938, during the Spanish Civil War, 795 prisoners escaped from the fortress and fled into the night. Unfortunately, most were rearrested and once again faced deplorable conditions. More than 200 people died, hunted by guards and executed. Only three men managed to successfully escape to France.

The prison closed in 1945. A monument to those who escaped in search of freedom now stands atop the mountain.