Researchers say a dozen of them are bad enough to cause explosions.
Last winter, researchers from Duke and Boston University got in a car and drove up and down all 1,500 miles of roadway in Washington, D.C. with a spectrometer and a GPS kit. Their study identified an alarmingly pervasive problem that's otherwise impossible to see: Some 5,893 spots in the city where aging and buried natural gas pipelines leak so much methane that it's possible to detect it above ground.
The leaks produced, on average, methane concentrations more than twice the level of background air samples collected around town. And, according to the study's findings, published this week in the journal Environmental Science & Technology, more than a dozen of these spots were giving off enough of the greenhouse gas to cause explosions.
In this second map from the paper, the worst leaks are shown in red spikes:
Lest you think you think you're safe driving (or walking) over manholes outside of D.C., the same researchers previously found similar problems in Boston.
The Washington study was originally conducted in January and February of 2013, and the researchers reported the worst leaks to the city. When they returned to measure those same spots again four months later, nine of them were still giving off dangerous levels of the gas. In the below video, Robert B. Jackson, a professor of environmental sciences at Duke who led the study, explains how the team produced that map and why our aging infrastructure should make us anxious: