The explosion in Harlem on Wednesday that leveled two buildings and killed at least two people was only the latest deadly explosion in New York City in recent years. While the reason the buildings blew up isn't yet known, there's good reason to consider old, cast-iron gas lines a primary suspect. Those gas lines run throughout the city, and — not to be alarmist! — could fail at any time.

In 2007, 69-year-old Kunta Oza was killed when her house in Sunnyside, Queens exploded. That explosion was traced back to a break in a gas main in the street outside — as the Daily News reported,  "an 80-year-old cast-iron gas main."

Both the age and composition of that main are significant. In a remarkable bit of timing, the Center for an Urban Future released a report this week detailing the age of various parts of New York City's infrastructure, and it included a look at the aging natural gas network. In the area of Wednesday's blast — 116th and Park Avenue — the gas provider is Con Edison. According to the CUF report, Con Edison's network has a number of trouble spots.

Their mains are 53 years old on average and 60 percent are composed of unprotected steel or cast iron, the most leak-prone material. According to the federal Pipeline and Hazardous Materials Safety Administration, Con Edison experienced 83 leaks for every 100 miles of main in 2012. Corrosion was responsible for a total of 427 of these leaks.
NYC.gov

Adam Forman, the author of the report, pointed out that cast-iron is more than the most "leak-prone" material. In 2011, after a series of natural gas accidents, the federal government created a program that would help municipalities move away from "cast and wrought iron, as well as bare steel" as they're the materials that pose some of the highest risks of rupture. "[T]he degrading nature of iron alloys, the age of the pipelines, and pipe joints design have greatly increased the risk involved with continued use of such pipelines," the Department of Transportation explained.

According to Forman's research, 58 percent of ConEd's 2,234 miles of gas mains pre-date 1960. Sixty percent are unprotected steel or cast iron. As for those leaks, they help contribute to the 2.2 percent of gas ConEd ships to customers each year that doesn't reach its destination. Two percent. Last year, Damascus Citizens for Sustainability issued a report documenting a series of tests around Manhattan to identify natural gas leaks. The maps that they produced are chilling. In November 2012, they surveyed a number of locations around Manhattan. The red lines on the map below correspond with the amount of natural gas they detected in the above-ground air. (The map is from the group's preliminary report.)

We don't know what caused the explosion in Harlem on Wednesday. According to Mayor Bill de Blasio, Con Ed was notified of a leak in the area 15 minutes before the explosion. Alfonso Quiroz, spokesperson for ConEd, told The Wire by phone that the area is "served by an eight inch low pressure gas main," but that they were "still trying to figure out what [the pipeline] was made of." Among the possibilities: plastic and cast-iron.

Update, Thursday: ConEd confirmed to Huffington Post that the main pipeline into the building that exploded "was partly made of cast iron and dated back to 1887."

A number of outlets have pointed to a work order at the location of the explosion from last June which involved extending a gas pipeline to a fifth-floor stove. Others have noted a number of building code violations from just last month — although most of those are unrelated to anything that would have caused any sort of explosion.

What we do know is that there are a number of old pipelines throughout the city that need to be upgraded, and that those upgrades are slow to happen. And we know what happens when they fail. In 2009, another house in Queens' Floral Park neighborhood exploded, a blast tied to a hole in the gas main across from the house. In the wake of that explosion, Joe Oza, the son of the woman killed in 2007, had one simple thought: "Here we go again."