Those suspicions came one step closer to being confirmed last week after the opening of a 75-acre underground complex was dug out from below the earth and granite used to seal off the entrance, the Times of Israel reports. The excavation team was led by Austrian filmmaker Andreas Sulzer, who says the site was "likely the biggest secret weapons production facility of the Third Reich"—a facility that probably relied on forced labor from the nearby Mauthausen-Gusen concentration camp and may have even been the testing location for a nuclear bomb, the Daily Mirror reports. The weapons facility was believed to have been manned by SS General Hans Kammler and situated near the B8 Bergkristall factory, where the first working jet-powered fighter was created, International Business Times reports; Sulzer first got wind of the site after seeing references to it in an Austrian physicist's diary. "Up to 320,000 inmates are said to have died because of the brutal conditions in the subterranean labyrinth," Sulzer tells the Sunday Times, per the Times of Israel. Those inmates were chosen for skills in physics, chemistry, or other sciences that would advance the Nazis' quest for WMD, Sulzer says. Digging at the site was halted by local officials who demanded a permit, but Sulzer says excavation will restart next month. "We owe it to the victims to finally open the site and reveal the truth," he says, per the Mirror.