In some places, the entry of ordinary people is strictly prohibited. Check out what they are

Metro-2, Russia

Also known by the code name D-6, Metro-2 is a secret underground transport system parallel to the Moscow public metro. 

Secret bunkers were created during Stalin’s time and are believed to connect several of the capital’s most important government and administrative institutions, including the Kremlin, the FSB headquarters in Moscow, Vnukovo-2 airport, the Security Council and the CIS anti-terrorist center.