Underground of Petrovaradin fortress
The underground system of corridors, galleries and halls is widely spread under the Petrovaradin fortress with about 16 km of underground corridors arranged in several levels. The underground system was built as a countermine system to defend the upper fortress and it is one of the largest ever built in Europe. The construction took place in 18th century from 1765 to 1783. There are four levels of defence. The system includes mine fields, chambers for soldiers and weapons. Chambers and corridors have thousands of loopholes for guns and canons. There are two wells for the water supply, and there is a whole system of ventilation openings.
Corridors and galleries are full of obstacles, traps and barricades in case if enemy soldiers penetrate the system. There are many crossroads and the whole system is a one huge labyrinth. All corridors have markings with names, signs and arrows for orientation. Many of them are 'blind corridors' with a dead end.
The underground is often mistakenly called 'Petrovaradin catacombs' but no one was really buried there. The purpose was strictly military. Even though many people did die while digging and building the counermine system. The whole venture was a huge effort even for the great Habsburg Empire. The whole thing was designed by Alfred Schroeder, a nobleman from Austria and a military officer.
In 1526 during the siege and eventually fall of the fortress, Turks used the mine attack tactics and took the fortress in just 48 days. This is why Schroeder's plan was accepted straight away. Underground corridors with defencive positions are arranged bellow the upper fortress and under the Hornwerk.
Mine galleries got their names from Habsburg rulers or noblemen: Jozefs Mine, Leopold Mine, Karls Mine, Rudolf Mine etc. Boards with names also included markings, and purpose of the corridor: Kom Gall, Ausgang, Arrond, etc. Roman numerals are marks for defencive level (I to IV) and Arabic numerals stand for the number of the corridor.
A small part of the system is open for tourists. Majority of corridors under the Hornwerk is accessible but it is easy to get lost without a guide. There is no light, and some parts are very humid with poor ventilation. Most of the corridors are well preserved but some are very much devastated with graffiti. Some parts are housing bats since people rarely walk down to disturb them.
The system of corridors is very much explored today. There are legends about a tunnel under the Danube, about hidden treasure and other but no one has ever found any of those. Ghost Hunters International was looking for ghosts here (you guessed, they found that the fortress was haunted) and there were also some stories about a giant snake and similar nonsense. The only real danger is that one can easily get lost inside without proper guidance.