Two buildings collapsed this morning in New York City's neighborhood of Harlem following what's believed to be a gas leak explosion, according to Mayor Bill de Blasio. It's unclear how many people might have been killed or injured, but early reports indicate that at least seven people are dead and another 70 are hurt. The buildings comprised 15 apartments, reports The New York Times, and many residents have yet to be located.

"Our community's 9/11"

Gas and electricity company Con Edison has acted by shutting off the gas in the surrounding area while New York City firefighters try to extinguish the flames. Public transportation has also been affected as the Metro-North train service was partially suspended. According to the National Transportation Safety Board's twitter account, one of its teams is investigating the scene.  "The only indication of danger came about 15 minutes earlier when a gas leak was reported to Con Edison," New York City's mayor said this morning after reaching the scene, calling the event a "tragedy of the worst kind." Congressman Charlie Rangel, who represents that area, reacted with similar emotion, stating that the explosion is "our community's 9/11."

So far, there has been no word on what might have caused the leak. Bob Ackley, president of Gas Safety Inc., says that the leak could have been the result of any number of events. "There could have been a leak inside the building due to criminal activity," he says, "or could also have been a punctured main from excavation." Ackley's best guess, however, is that the explosion was caused by a break in a cast-iron main, which he calls "a ticking time bomb." This is a likely scenario because, according to a ConEd briefing issued this afternoon, the buildings were served by an 8-inch main made of plastic and cast-iron.

When the earth moves, cast iron pipes can crack

If the 8-inch main proves to be the source of the leak, then the advent of warmer weather might be the reason it cracked. "Right now we are in a critical period because the frost is coming out of the ground and the earth is moving," he says. When the earth moves, either because of events such as an earthquake, a broken water main, or emerging frost, cast-iron pipes are susceptible to cracking. "A hairline crack" is enough to cause a very hazardous situation, he says. After that, all you need is a spark from a match or a lighter. "So you have to be aware. If you smell gas, you call it in."

When gas leaks from a main, an explosion can happen quickly. All it takes for gas to enter a building is a conduit, which can be anything from a broken telephone line that goes straight into a building to cracked pavement. It really doesn't matter: if there's a way to get into a building, the high-pressure gas contained in a broken main will find it. As Popular Mechanics reports, "a buildup of gas in a trapped environment elevates the risk for explosion." Since gas is a combustible, it's perfect for heating a building or cooking — you only need to burn a small amount to generate a lot of heat. Unfortunately, it's that same property that makes it prone to causing massive explosions like the one in Harlem.

New York City's infrastructure is aging

Of course, the fact that New York City's infrastructure is aging probably didn't help the situation. According to a report recently released by the Center for an Urban Future, 35 percent of Con Ed's 2,234 miles of gas mains in the city are made of either cast iron or unprotected steel that predates 1960. Newer gas mains are usually made of either plastic or coated steel, both of which are far safer than cast-iron pipes, according to Ackley. Plastic doesn't crack the way cast-iron mains do, and coated steel is great because it inhibits corrosion, he says. Coated steel also has a longer life-span.

So what would it take to improve New York City's gas pipes? "I hear numbers of about a million dollars per mile," Ackley says, "so you are looking at almost a billion dollars" just to replace the cast-iron mains. The number could be even higher because of Manhattan's underground. "It's just full of utilities," Ackley says, that workers have to find a way around.

Ackley cautions that it might be months before we know what happened. "Sometimes it takes years," he says. "But if Con Ed finds a broken gas main, that will go a long way to indicating the cause." For now, "we just don't know."

This story was updated after publication to include additional details provided by authorities.