 Complete the following phrase:  From 1943, the purpose of the camp was switched from quarrying to armaments production in vast underground factories, to protect the industrial output from Allied bombing. Work on the tunnels was begun by the Kellerbau Kommando at the original Gusen camp, which had a high mortality rate. The tunnels at Gusen were initially used for the production of Messerschmitt Bf 109 fighter aircraft. The work took on new urgency after the bombing of the Messerschmitt plant in Regensburg on 