Besides the main camp Dora, housing an average of 15,000 prisoners, the main subcamps were ' (established 2 May 1944, averaging around 8,000 prisoners), ' (1 April 1944, 4,000 prisoners), "" (13 March 1944, 1,000 prisoners) and the SS construction units III and IV (totaling around 3,000 prisoners, distributed among several small camps along a newly constructed railway line between Nordhausen and Herzberg am Harz. More subcamps were added once Mittelbau was officially independent. By spring 1945, the number of inmates totaled over 40,000 in around 40 camps.