Considering what may be underneath, it's darkly ironic that Berlin's highest artificial hill is called "Teufelsberg," or "Devil's Mountain."

"We know for sure that there's a massive multi-story bunker complex underneath," Dietmar Arnold, co-founder of the Berlin Underground Society, a group dedicated to researching subterranean urban history, said in an interview with German news agency dpa.

Arnold and his colleagues are eager to explore what he called "the last undiscovered secret that underground Berlin has to offer."

Rubble mountain

When World War Two came to an end in 1945, the devastated German capital began to clean up and rebuild. Berliners had to do something with the millions of tons of rubble their city had become, so they piled it up just outside the city.

The result was a nearly 120-meter (390-foot) peak, covering up what would have been a key element in Adolf Hitler's plan to create the powerful "Germania" superpower.

In the late 1930s, Nazi architect Albert Speer was commissioned to design an elite military academy on the site of what used to be a popular lake-side vacation destination near Berlin.

Though construction of the academy began, the Nazis had to interrupt the building plans a few years later as the war heated up and resources were needed elsewhere. It was never completed.

Uncovering history

During the Cold War, the Americans set up a radar unit on what had then become Teufelsberg to listen in on radio communications in East Germany. The buried Nazi academy was quickly forgotten -- until the Berlin Underground Society recently discovered documents pertaining to the site.

Arnold said close to 1,000 underground bunkers had been built in Berlin during the Nazi era, though many were later destroyed. Founded in 1997, his organization has already uncovered 50 of them. One of their most significant projects was a digital reconstruction of the bunker where Hitler took his own life in April 1945.

Although efforts were supposedly made to blow up the half-built military academy at the end of the war, Arnold is convinced that his group will find it intact -- a find that would not only be a significant feather in their caps, but also an important lead for World War Two historians.