Metro-2 in Moscow, Russia, is the informal name for a purported secret underground metro system which parallels the public Moscow Metro. The system was supposedly built, or at least started, during the time of Joseph Stalin and was codenamed D-6 by the KGB. It is supposedly still operated by the Presidential General Directorate of Special Programmes and Ministry of Defence.
The length of Metro-2 is rumored to exceed that of the public Metro. It is said to have four lines, and to lie 50 to 200m 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 1994, the leader of an urban exploration group, the Diggers of the Underground Planet, claimed to have found an entrance to this underground system. The Metro-2 rumors have been dismissed by one source as "a parody excursion by public transport into the murky world of 'intelligence'". This source describes virtually all available information as "speculative, unsupported by documentation such as photographs. There are narratives told by people who said they helped build Metro-2, and urban spelunkers claim to have 'seen' Metro-2, but there are no explicit 'firsthand' accounts."
Read more about Metro-2: Origin of The Term, Confirmed Information, "Underground City" in Ramenki