There are many underground structures located in the Owl Mountains, previously Germany, now territory of Poland. In 1943-45, they were parts of the massive Nazi project code name “Riese” (the Giant). The purpose of Riese remains a mystery. The suggested explanations include a future weapons factory and military headquarters. The entire workforce of the project was the slave labor of POWs and concentration camp prisoners. In the mountain slopes, they drilled holes and used explosives. Workers removed the all the rocks by hand, by loading them on carts. Working 14-16 hours a day, prisoners created tunnels and large underground halls. Network of new roads and narrow gauge railway connected individual structures. A new electrical, water and telephone infrastructures supported the whole project. It is estimated that not all of the underground tunnels built for Riese were discovered yet.