A filmmaker has found a secret network of underground bunkers used by the Nazis to develop an atomic bomb.

Located in the hills surrounding the Austrian town of St. Georgen an der Gusen, the vast site covers an area of up to 75 acres. It is believed that it could be connected to the nearby Bergkristall factory, an underground facility where Nazi scientists and engineers developed the Messerschmitt ME 262, the world’s first jet powered fighter plane.

The Bergkristall factory was discovered and inspected by Allied troops after the war, but they appear to have missed the secret complex. After the war the underground complex of tunnels was abandoned, and for safety reasons filled in with concrete by local authorities ten years ago.