For millions of Muscovites who daily descend up to 275 feet on crowded escalators, to then be carried across their vast city in often-crammed wagons hurtling through black tunnels, the safety and clockwork-like efficiency of their legendary metro has been an article of faith.
Terrorists have targeted the metro twice in the past decade, with tragic consequences. But until today there hadn't been a mass-casualty accident on any of the city's 200 miles of underground track for decades.
At the height of morning rush hour Tuesday, in the very deepest tunnel of the metro system, a train moving at about 45 mph suddenly derailed. At least three crowded carriages were smashed, killing at least 21 people, injuring over 150, and leaving dozens trapped far below the surface for hours.
Eyewitness accounts and mobile phone photos and videos posted on social media sites describe the horror of the crash and the scramble of survivors in the dark tunnel, nearly 300 feet beneath one of Moscow's busiest thoroughfares.
The Moscow metro opened 80 years ago and is one of the world's largest -- with 180 stations that serve almost 10,000 trains daily.
Many of the older stations in downtown Moscow were designed to be "proletarian palaces," with marble platforms adorned with Soviet-themed statues, frescoes, and mosaics that are still prime tourist attractions for visitors to the city.
About half of the stations were originally designed to double as nuclear fallout shelters, and tour guides enjoy pointing out the the places where giant blast doors are still coiled up in recesses behind tunnel walls, ready to be rolled out.
The metro has been undergoing rapid expansion, in part as an official answer to Moscow's often paralytic traffic jams. Tuesday's accident occurred on one of the metro's newest lines, which would seem to rule out aging infrastructure as a cause.
Officials say several potential causes are being investigated, including an electrical surge, a carriage defect, a flaw in the tracks, or a switch failure.
In line with its usual response to such events, the Kremlin's Investigative Committee, Russia's top police body, immediately launched a criminal investigation and promised that the guilty parties would be found.
"As it is a man-caused accident, it is obvious that there are people responsible for it, so soon there will be suspects in the case," spokesman Vladimir Markin said in a statement published on the Committee's official website.
Coverage of the accident does suggest a fast and effective response by Russia's Ministry of Emergency Services [MChS], one of the country's few government departments that enjoys widespread social respect, which quickly closed off the road above and brought in helicopters to evacuate the injured.
"I have to say that MChS has been steadily improving its work over past years, and it really shows in a case like this," says Yury Kobaladze, an independent security expert.
Experts are divided over how badly public confidence will be shaken by the accident.
"The question that will occupy peoples' minds is, how could this have happened? Our metro was always considered one of the safest ways to travel," says Gennady Gudkov, a former Duma deputy and Kremlin critic. "But few people noticed that little accidents and delays have been piling up lately. Lines would stop working for a few hours, unexplained glitches would happen. I have the impression that problems were accumulating that weren't addressed," he says.
But Dmitry Oreshkin, head of the Mercator Group, an independent Moscow media consultancy, says the public will probably move on from this tragedy without looking back.
"Of course people have fears, but what's the alternative? You have to go to work," he says. "The metro was hit by terrorist acts in the past, but pretty soon people were back in the metro, fears forgotten, as usual. This will be the same."