Moscow's ageing Soviet metro system became the unlikely scene for sexual antics this week as a thrill-seeking young couple crept into a tunnel and made love on the tracks.
Moscow has a subculture of enthusiasts who explore its maze of tunnels including the metro system, called "diggers"
The metro ordered an urgent security sweep on Friday after what appeared to be the first such incident in the subway famed for its ornate stations hung with chandeliers.
Moscow has a sub-culture of enthusiasts who explore its maze of tunnels including the metro system, called "diggers", and a young couple this week published photos of themselves romping naked on the rails.
Their feat came to light only after the couple posted their photographs on social networking sites, prompting questions about the ageing metro's security.
"This is the first time we have seen something like this. We don't have experience of this sort of thing," a metro spokesman told AFP.
-'Breach of security'-
The metro management announced spot checks of the security on the whole system to see if it was possible to gain illegal access to the underground zones, the spokesman said.
"In particular, all the ventilation shafts will be checked to see if they can be used to access the underground system," he added.
It appeared to be the first such rendez-vous to come to light in the history of the 79-year-old system, which is used by some eight million people every day as the best way to beat the jams.
The couple picked a service tunnel that is only used to divert trains in an emergency, not one along which metro trains rumble up to every 90 seconds at peak times, the metro spokesman added.
"If such an incident had taken place on a constantly operating stretch of track, the wrongdoers would have been discovered and caught immediately, as soon as they entered the tunnel."
Any breach of security is alarming for the metro given that it experienced suicide bombings with multiple casualties in 2010 and 2004.
A member of the public chamber which advises the Kremlin even called for the metro security chief to be sacked.
"It's a good job these young people were just having fun and not plotting anything bad," Dmitry Chugunov, head of the chamber's public safety commission, told pro-Kremlin Izvestia daily.
"Now what's needed is to look not for them, but the holes they used to get onto the tracks."
Police said on Thursday that they were carrying out a check into the incident and trying to establish the identity of the couple.
-'We are not the first'-
The male digger, who only gave his name as Andrei R., told Life News website that on Tuesday he and his girlfriend entered a service tunnel on the green metro line, which runs close to the Kremlin.
"We were definitely not the first," he said.
The man was identified by REN TV as an Internet user with the nickname Andrey Rayonny, who regularly posts pictures on Instagram.
He is part of the "digger" movement, in which urban explorers descend into tunnels built under Moscow, including secret Soviet-era installations.
The authorities are largely powerless to stop them as there are believed to be many illicit ways to get in.
The Moscow metro is also the setting for another risk-taking trend in which young Russians attempt to ride between carriages or on their roofs as they travel between stations.
These extreme thrill-seekers called "train surfers" are taking a huge risk.
Last year, a young man was crushed between carriages on the busy circle line, while in 2011 two students died while riding on the roof of a metro carriage.
"The problem is not the metro, the problem is people," said the metro spokesman said.