Project Riese
Riese [ˈʁiːzə] (German for "giant") is the code name for a construction project of Nazi Germany in 1943–1945, consisting of seven underground structures located in the Owl Mountains and Książ Castle in Lower Silesia, previously Germany, now a territory of Poland.
None of them were finished; all are in different states of completion with only a small percentage of tunnels reinforced by concrete.
The purpose of the project remains uncertain because the 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 (HQ) and arms industry but comparison to similar facilities can indicate that only the castle was adapted as an HQ or other official residence and the tunnels in the Owl Mountains were planned as a network of underground factories.
The construction work was done by forced labourers, prisoners of war (POWs), and prisoners of concentration camps, and many lost their lives mostly as a result of disease and malnutrition.
In the presence of the increasing Allied air raids Nazi Germany relocated a large part of its strategic armaments production into safer regions including the District of Sudetenland. Plans to protect critical infrastructure also involved transfer of the arms factories to underground bunkers and construction of the air-raid shelters for government officials.
In September 1943, Minister of Armaments and War Production Albert Speer and the senior management of Organisation Todt started talks on Project Riese. As a result, the Schlesische Industriegemeinschaft AG (Silesian Industrial Company) was created to conduct construction work. In November collective camps (Gemeinschaftslager) were established for forced labourers, mainly from the Soviet Union and Poland, POWs from Italy, the Soviet Union, and later Poland as an aftermath of the Warsaw Uprising (List of camps).
...
Wikipedia