This is interesting
Here is number 7
Metro-2 in Moscow, Russia has a secret underground metro system which parallels the public Moscow Metro. The length of Metro-2 is rumored to exceed even that of the "civil" (i.e. public) Metro. (It is said to have 4 lines and lie 50 to 200 m deep. 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. In the late 1940s Stalin had created the tunnels in the event of a nuclear war. In 1994, a group of urban diggers had stumbled on to the underground system. Though not much more information is known known to the public about this.