A full secret underground system lies in tandem to the official metro beneath Moscow, according to intelligence reports and Russian insiders.
For many people visiting the Russian capital, its metro system is a real highlight.
The extensive network of rails link up some truly beautiful stations in which cavernous halls cut into the rock are decorated with vibrant colour schemes and chandeliers.
What is not on the Moscow tourist trail is the Metro-2, a secretive underground train system which has been the subject of speculation in Russia for decades.
The legend goes that Joseph Stalin commanded his inner circle to have the system built as a means of evacuating the country's top brass in the case of a war.
It is said to have four lines lying between 50 to 200m deep, connecting the Kremlin with the FSB headquarters, the government airport at Vnukovo-2, and an underground town at Ramenki.
How the enormous engineering project was carried out beyond the gaze of Moscow's citizens and off the pages of official public record underlines quite how much control the Soviet high command had over its people.
The fact that such a project was embarked upon at all shows how serious the threat of nuclear strikes were taken during the Cold War.
The true extent of the underground network has been hotly debated since 1992, when Vladimir Gonik's novel Preispodniaia about an underground bunker in Moscow was published.
In an interview about the book the author said it had been written over the course of 20 years and was based on information given to him by officials.
Gonik would later say that the network included a huge bunker with a 180m 2 apartment dug into the rock for each member of the Central Committee, including a living room, kitchen and bedroom.
Two years later urban exploration group the Diggers of the Underground Planet claimed to have found an entrance to the system, Unknown World reported.
Russian journalists have reported that the existence of Metro-2 is neither confirmed nor denied by the FSB or the Moscow Metro administration, which has only heightened speculation.
So to has a report from the US Department of Defence that devoted several pages to the metro and included a diagram of the system.
"The Soviets have constructed deep-underground both in urban Moscow and outside the city," the book states.
"These facilities are interconnected by a network of deep interconnected subway lines that provide a quick and secure means of evacuation for the leadership.
"The leadership can move from their peacetime offices through concealed entryways in protective quarters beneath the city.
"There are important deep-underground command posts in the Moscow area, one located at the Kremlin.
"Soviet press has noted the presence of an enormous underground leadership bunker adjacent to Moscow State University. These facilities are intended for the national command authority in wartime."
Several senior Russian officials have hinted that rumours of the underground's existence are true.
In 2008, Mikhail Poltoranin - a minister under Boris Yeltsin in the early 1990s - said that there was "an extensive network of tunnels and an emergency command centre in case of war, where you can command the nuclear forces of the country.
" It can hide a lot of people."
Former deputy chief of the Moscow Metro, Dmitry Gayev, has said he would be "surprised if it did not exist" when asked about Metro-2.
Others have been quicker to pour cold water not on the idea as a whole, but the apparent extent of it.
Vladimir Shevchenko, who has advised all the Russian and Soviet leaders from Mikhail Gorbachev onwards, has said Metro-2 consisted of a single underground railway line.
He claimed it ran from the Kremlin to a summer home in Volynskoye.
According to the Moscow Times, urban explorer and Youtuber Andrei Pyzh was sentenced to five years in prison in 2021 for 'sharing state secrets' when he took information about Metro-2 to Ukraine.
In 2008 the head of the Moscow Metro independent trade union, Svetlana Razina, said drivers were recruited for work on the secret lines several years before.
"Entering the midst of these tunnels is only for people with special clearance," he told one Russian publication.
"Most often, these branches used very short trains, consisting of battery-electric locomotive and one passenger car."
Whatever the extent of the network, the full truth is likely never to be known.
Oleg Gordievsky, a former colonel of the KGB who defected to MI6 in the 1980s, told magazine Argumenty i Fakty in 2001: "You still do not know the main KGB secret yet: a huge underground city, a whole communications network of such facilities. But they will not show you; they will never, of course."
