Mittelbau was a complex of factories, storage depots, facilities and prisoner camps, some underground, that were used from August, 1943 until April, 1945 to manufacture and test the V-2 rocket near Nordhausen in central Germany. The main V-2 assembly line was located in an underground factory called Mittelwerk that was excavated beneath Kohnstein Mountain, about 2 km southwest of the town of Neidersachswerfen. The Dora concentration camp (later called Mittelbau KZ), where most of the Mittelwerk workers were imprisoned, was located on the southwest edge of the Kohnstein, near the southern portals of Mittelwerk tunnels A and B.
http://www.v2rocket.com/start/chapters/mittel.html
http://www.dora.de/media_en/index.html