Portsmouth naval base is the oldest in the Royal Navy, and it has been an important part of the Senior Service's history and the defence of the British Isles for centuries. At one time it was the largest industrial site in the world. It is home to one of the oldest drydocks in the world. The former Block Mills are of international significance, having been the first factory in the world to employ steam-powered machine tools for mass production. In 1985 a partnership between the Ministry of Defence and Portsmouth City Council created the Portsmouth Naval Base Property Trust to manage part of the historic south-west corner of the Naval Base, under a 99-year lease, as a heritage area: Portsmouth Historic Dockyard. It allows members of the public to visit important maritime attractions such as "Mary Rose", and .