The concept of “closed city” appeared in the USSR in the late 1940s and had a strict purpose: to serve the interests of large industrial groups, the army, or major research institutions. These are areas of production or extraction of minerals and natural gas in large quantities. It was not worth seeking these “economic boosters” on Soviet maps because they were not included. Simply designated by postal code, the closed cities were kept secret for long periods of time. Today most of them are known.
Russia has 42 of them. Together with the collapse of the Soviet State, they ceased to be secret, but it is likely there are more, unknown yet. They are called “ZATO” (closed administrative territorial entities). Surrounded by barbed wire, protected by armed guards, the entry of these secret areas is strictly forbidden to foreigners. Such prohibited cities are located in remote areas, without any official way to access them. To enter there, the Russians must have special permission. Moving out of these closed cities is also not easy, as you need a special document from the authorities. Weirdrussia.com