Located 80km to the south of Wroclaw, Project Riese is a massive underground tunnel system that lies beneath the Owl Mountains – the oldest mountain range in Poland. However, to make sure that it withstood the test of time, the Nazis reinforced the walls using steel and concrete. Unfortunately, they couldn’t complete the reinforcement project as the Nazi soldiers abandoned the project during the war’s last days. Built during the Second World War between 1943 and 1945, Project Riese is reputed as one of the world’s largest military enterprises today. It is still unclear why the Nazis built this underground complex; neither is its exact size known. Some say the…