BELGRADE (Reuters Life!) - Only a few patrons enjoying drinks at Belgrade's Underground club know that a Medieval mass grave lies just beyond the bar's walls.
To repel Turkish invaders in 1440, Christian defenders of the city used a giant gunpowder mine to blow up hundreds of attackers digging tunnels beneath the city. Turkish bodies remained in the rubble that sealed one of the tunnels, meters (yards) away from the club's bar.
"I know this place is old and sometimes eerie, but I only recently read a history book about what happened here," said student Ivana Jovanovic, 23.
Over the centuries, attackers and defenders have carved a vast network of underground tunnels, fortifications, storage areas, command posts and bunkers. Today, only the occasional tourist, archaeologist or historian knows about the hidden world below Serbia's sprawling capital.
"We explored and mapped about 15 km (9 miles) and have only scratched the surface," said Vidoje Golubovic, a Belgrade-based historian who occasionally hosts guided tours for enthusiasts.
The city's tumultuous history accounts for the unusually rich honeycomb of tunnels below.
It was just a decade ago that Belgraders most recently went into underground basements and bunkers, this time to avoid NATO bombs during the war involving Kosovo.
In Belgrade's more than 1,000-year history, Celts, Romans, Huns, Slavic tribes, Byzantines, Hungarians, Ottoman Turks, Serbs, Austro-Hungarians and Germans have tried to dominate the strategically important confluence of rivers Sava and Danube, considered the gateway to Europe.
Underground Belgrade usually comes to light only during construction when heavy machinery breaks through the brick walls or limestone caverns, sometimes even exposing waterways dating back to Roman domination of the Balkans.
"Wherever one digs, there's a probability of hitting a tunnel of sort. Belgrade's hollow like Swiss cheese," Golubovic said.
Most of the tunnels, wine cellars, underground bakeries, wells, and a World War Two German command outpost in the Kosutnjak neighborhood overlooking the city are now abandoned and closed to the general public.
Pushing open a rusty door, just steps from modern apartment blocks in the Kosutnjak neighborhood, Golubovic lowered himself down rusty ladders to a dark hallway where steel bed frames were still attached to walls.
"From this vantage point, Germans had perfect command of the area," he said.
A few World War Two veterans still remember the menace of Belgrade's underground world.
On October 19, 1944, groups of German troops retreating from the advancing Yugoslav Partisans and the Soviet Red Army vanished inside a building in central Belgrade.
"They went deep down, to sewers and tunnels," said Milisav Lukic, 85, a commander in the then Yugoslav communist People's Liberation Army.
Belgrade was officially liberated on October 20, 1944, but for weeks afterwards German soldiers would occasionally pop out of tunnels to fire off bursts of gunfire or toss hand grenades and then vanish.
Some 50,000 tourists visited Belgrade last year, but none had a chance to see the underground, not even the fortified chambers and dungeons under the Kalemegdan fortress, perhaps the city's best-known attraction.
"A great way to attract tourists is to put its historical sites to the best possible use and few things are better than the fort and Belgrade's underground," said Ljubodrag Stanojevic, owner of the upscale Kalemegdan Terrace restaurant which is built into the old fortifications.
A star-shaped Baroque fortress designed by engineers Andrea Cornaro and Nicolaus Doxat, Kalemegdan is a mixture of 17th century Austrian fortifications, 14th century Serbian and Turkish towers and Roman walls.
"Only a fraction of tunnels, depots, dungeons and dugouts, under the fort are explored," Golubovic said.
City authorities blame property issues, bureaucracy and general lack of historical interest for the general ignorance about underground Belgrade.
"We want to do something with that, but we are facing numerous problems to obtain permits. We were also told that some areas are unsafe," said Jasna Dimitrijevic, head of Belgrade's tourist department.
The Defense Ministry's Military Museum that operates a part of the fort occasionally allows tourists to visit some of its underground chambers.
"I played in the tunnels inside the fort as a kid and I can only imagine how interesting that might be for thousands who would like to see it. Not to mention profitable," said restaurateur Stanojevic said.
In Zemun, a section of northern Belgrade, homeless people live in tunnels dating back to Austro-Hungarian rule.
"It is dry even in winter, with few rats and bats and other crawly creatures," said Milos, 56, whom a visiting reporter found while exploring a small tunnel in the area.
Editing by Adam Tanner and Paul Casciato