The name of this forgotten place is Object 825 GTS, a top secret Cold War nuclear submarine underground base located in the Balaklava Bay, Crimea, Ukraine.
The Soviet military started to build the base in 1957 under the directive of Joseph Stalin. Stalin commissioned his crony Lavrentiy Beria to find places for secret nuclear submarine bases. In the case of a nuclear attack, the submarines stationed in this base would launch an strike against the United States and its allies. Beria's team thought that Balaklava was the perfect place, near the Sevastopol naval base, which still in use by the Black Sea Fleet.
120 tonnes of rock were removed in the middle of the night, using barges that dropped the material at sea. The resulting complex was used to dock and fix submarines, load them with nuclear weaponry and torpedoes. Inside the base, 3,000 men worked in the submarines until it closed in 1995, following the collapse of the Soviet Union. Years later, the complex was turned to the Ukranian Navy, who turned it into a museum. [English Russia]