No.5: Mezhgorye (Russia)

Source : Wikipedia
Mezhgorye is a “Closed City” located in the Republic of Bashkortostan, Russia. Entry is strictly prohibited not only for the general public but even for nearby residents. Founded in 1979 near Mount Yamantau in the Ural Mountains, the city is rumored to house nuclear missile silos and underground bunkers dating back to the Soviet era.
The Russian government has never disclosed details, but it is believed to serve as a strategic nuclear facility and a hub for classified military projects. Security checkpoints are positioned at the town’s entrances, and unauthorized individuals are rigorously denied access. Often referred to as “Russia’s Area 51,” Mezhgorye remains shrouded in mystery, with speculation about vast underground complexes continuing to fuel intrigue.
No.4: Diego Garcia Island (British Indian Ocean Territory)

Source : Wikipedia
Located in the heart of the Indian Ocean, Diego Garcia Island was once home to indigenous inhabitants until the late 1960s, when they were forcibly relocated by the U.S. and U.K. governments. Today, the island serves as a massive U.S. military base.
The entire island is designated as a military Restricted Area, with civilian access strictly forbidden. Not even former residents are allowed to return. The British Indian Ocean Territory government officially states that Diego Garcia is not a tourist destination and that prior permission is required for any travel. There are no commercial flights to the island, which remains strategically vital since the Cold War. Known as the “Fortress Island,” Diego Garcia is effectively erased from maps and remains under tight security.