A SECRET underground Nazi complex that was never completed remains a mystery as no one knows for certain why Adolf Hitler's genocidal regime built it. The Third Reich began to build a gigantic subterranean bunker with Jewish slave labour in what was then Germany at the end of WW2 - known as Project Riese. 13 The entrance to the secret Nazi underground tunnels in Poland Credit: Alamy 13 The tunnels were built at the end of the war but abandoned as the Soviets advanced Credit: Alamy 13 Some believe the complex could have been a new HQ for the Nazi...