The Postojna Cave is one of the Slovenia’s most popular tourist destinations and one of the most famous caves in the world. Each year, it attracts half a million visitors from around the world eager to explore the cave’s vast underground chambers.

When they exit the cave, some visitors may notice that the walls near the cave’s opening are unusually dark. That’s because it was in this part of the cave where one of the greatest acts of sabotage in Slovenian history took place during World War II.

After it had occupied Slovenia following Italy’s surrender, the German army was looking for a place where it could store the vast amounts of fuel it needed for military operations in Northern Adriatic region. The Postojna Cave was an ideal choice. Closed to tourists because of the war, it offered complete protection from Allied airstrikes.

The Slovenian Partisan resistance was aware of the fuel stores and decided to sabotage them in order to weaken the German enemy. But there was a problem: The cave was under heavy German guard, making it impossible for anyone to sneak by. The Partisans, however, knew something that the Germans did not: there was another way into the cave.

The old passageway that led into the cave wasn’t on any official maps and was in poor shape, but on April 23, 1944, the Partisans of the Vojko Brigade managed to make their way through it -- and entered the cave. They set fire to the stored fuel, causing a massive explosion that stripped the stalactites from the cave’s ceiling. The wily saboteurs made it out alive through the same passageway they used to get in. The fire continued to burn for another week, and Germans were left without their much-needed fuel.

The Partisans’ exploits were eventually celebrated in a Yugoslav movie known as The Sole Exit. Like most films, it dramatized parts of the story, but it ensured that the exploits of the Vojko Brigade have remained vivid in the public imagination. Today, soot on the cave walls and the absence of stalactites still bear witness to that fateful April day when German forces were dealt a devastating blow in one of the world’s most famous caves.