Petrovaradin Fortress lies on the right coast of the Danube river, where the picturesque old town of Petrovaradin meets Serbia’s second largest city of Novi Sad. The fortress, whose construction started in 1692, was an important strategic point throughout history. The strategic importance of the fortress has earned it the nickname “Gibraltar of the Danube.”

Though it’s a popular site, most tourists are not aware of the secret underground tunnels hidden beneath the fortress. The underground military galleries—often referred to as the catacombs—create a four-story, roughly 10-mile-long (16 kilometers) network of passageways.

There are numerous artifacts and symbols found on the walls, such as the Maltese cross and various masonic symbols. One of them is the ipam miam inscription, used by the alchemists and Freemasons. Each level is marked with a different color, with the fourth (black) one being the most mysterious, as on its bottom lies the so-called “Kaiser Well,” from which the Austrian king Joseph drank water.

There are many legends related to the fortress’ tunnels, including a myth about hidden treasure. The story goes that the Austrian Imperial Treasury was brought into the fortress to hide it from the Napoleon’s possible attack on Vienna, and that some of the treasure has still remained hidden underneath.