Due to allied bombing, the Nazis decided to set up an underground rocket assembly factory near the Buchenwald camp in 1943, using slave labor to dig the tunnels. It was in operation for about 18 months, during which about 60,000 prisoners passed through. About 20,000 died, predominantly from exhaustion, starvation, disease, and cruel treatment. The camp was liberated by US Army units on April 10th, 1945.  Link