The explosion that destroyed two buildings in New York City earlier this month brings new attention to a risk experts have been warning is especially great beneath the oldest U.S. cities: natural gas leaks, according to an article in National Geographic.

Eight people were killed and more than a dozen were injured when the explosion, apparently caused by a natural gas leak, destroyed two apartment buildings in the city’s Harlem neighborhood.

“The explosion comes at a time when utility companies in many parts of the United States are grappling with a difficult problem: what to do about aging natural gas mains, many of them in difficult-to-access locations in older urban areas, which have deteriorated and are prone to leaking,” the article states. “The gas mains that are cause for most concern are made of cast iron, many of them dating back to before World War II.”

Old cast-iron pipes deliver much of the natural gas in American cities and metropolitan areas, including Boston, New York, Philadelphia, southwestern Connecticut, northern New Jersey and Chicago, according to National Geographic. Some of those cities also have high rates of “unaccounted for” gas, which could be due to leaks.

Recent studies have revealed thousands of natural gas leaks in the streets of Boston and Washington, D.C. Some leak sites had underground methane concentrations as high as 500,000 parts per million, ten times the threshold at which an explosion can occur. “If there’s a telecommunications manhole there, and you get a spark or a short, the air can ignite,” Duke environmental sciences professor Robert Jackson, who led the study, explained in an interview after publishing the study. “If someone drops a cigarette butt down a manhole, it can do the same thing.”

“Our study highlights the systemic nature of these leaks,” he added. “People don’t understand how pervasive they are.”

Some of the biggest problems are in New England, where Rhode Island (27 percent of mains), Connecticut (19 percent), and Massachusetts (18 percent) all have high concentrations of iron in the ground, according to the article.

To read the National Geographic article, click here.