Dora is underground factory for serial production of  V-2 rockets, V-1s and jets engines. It's situated in middle of Germany in Harz mountains.Mittelwerk company was provider of V-2 rockets. Rockets were produced in halls 21 till 41. Last image is scheme of factory. As far as I know the most of underground is now closed to public. But there is still lot of combustion chambers and engines, especially in hall No.29.