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In the late 1940s, cities in the Soviet Union began to disappear from official maps. Locations were erased from statistical documents and if you tried to venture to one yourself, you came up against a checkpoint. Foreign citizens were completely banned from these areas, some Russian citizens were turned away as well.
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These were the closed cities of Russia, also called secret cities. They were formed to forward weapons development and manufacturing, including chemical, biological, and nuclear research. Referred to in Russian as "Naukograds" (Science Towns), these cities contained high concentrations of scientists and researchers. Some were focused on academic pursuits, others were the hubs of technological creation, building sputnik, warheads, and long-range missiles.
Many of these cities have been reopened in the decades since their founding but an estimated 44 still exist. To this day, foreigners are not allowed inside these places and there are areas within them that even residents of the city are forbidden to enter.
Read more about Russia's secret cities:
The Calvert Journal: Hide and seek: a rare glimpse into one of Russia's last closed cities
GlobalSecurity.org: Secret Cities