Experts in Berlin's wartime bunkers have announced the discovery of a forgotten Nazi military school buried under a man-made hill on the western edge of the city.

The army academy, designed by Nazi architect Albert Speer, is encased in the Teufelsberg (Devil's Mountain), a 116-metre (380ft)-high mound in Berlin which was constructed from the 26m cubic metres of the capital's wartime rubble.

The unfinished building, for which Nazi leader Adolf Hitler laid the foundation stone in 1937, was meant to become part of Germania, the huge capital of the 1,000-Year Reich. But "war-specific" problems, according to an internal Nazi memorandum, caused building work to be stopped just three years later.

The British occupation forces planned to turn the building into their headquarters, until it proved too difficult to convert. It was also too sturdy to demolish. Instead, half of Berlin's rubble - equivalent to 400,000 buildings - was poured on top, along with grass-seed, and so the Teufelsberg was born.

It has been a favourite place for Berliners to ski and sled in winter and fly kites in summer. The existence of the military college had been virtually forgotten by all but a handful of enthusiasts, until the Association of Berlin Underworlds discovered documents pointing to the academy.

"Most of the faculty must still be intact, despite the attempts to blow it up," said Dietmar Arnold of Underworlds. The association, which has found 50 bunkers in its 10-year existence, is keen to dig down. "We know for sure that underneath there is also a massive multistorey bunker complex."