Metro-2
Metro-2 in Moscow, Russia is a purported secret underground metro system which parallels the public Moscow Metro. The system was built supposedly during (or from) the time of Stalin and codenamed D-6 by the KGB. The length of Metro-2 is rumored to exceed even that of the "civil" (i.e. public) Metro. It is said to connect the Kremlin with the FSB headquarters, the government airport at Vnukovo-2, and an underground town at Ramenki, in addition to other locations of national importance.