Project Riese was the code name for an unfinished construction project undertaken by Nazi Germany between 1943 and 1945. It consisted of seven underground structures in the Owl Mountains and Książ Castle – pictured here – in Lower Silesia (now part of Poland). The purpose of the project remains uncertain because of a lack of documentation. Some sources suggest that all the structures were part of the Führer Headquarters; according to others, it was a combination of headquarters and arms industry, but comparison to similar facilities can indicate that only the castle was adapted as a headquarters or official residence, and the tunnels in the Owl Mountains were planned as a network of underground factories.Photograph credit: Jarosław Ciuruś