Initially a subcamp of Buchenwald concentration, it became an independent concentration camp in 1944 and supported a large network of subcamps within the Harz mountains . Due to air raids in the surrounding areas, inmates were forced to dig huge tunnels into the mountains to carry out forced labour within underground factories. In 1943, these factories produced V-2 missiles. The mortality rate at Dora-Mittlebau was higher than other camps, as prisoners were enclosed underground in extreme conditions; more than 1 in 3 inmates died there. Most of the prisoners in Dora-Mittlebau were sent on death marches in the last days of the war. Over 60,000 prisoners were sent to Dora-Mittlebau.