The phrase “aging infrastructure” generally brings to mind deteriorating roads and bridges. That’s reasonable, because roads and bridges are the most visible part of the nation’s infrastructure — and they’re often in rough shape these days.

Infrastructure also means ports, locks on major rivers, airports and train tracks. Those, too, are visible, and also needing investment to keep the United States globally competitive.

But there’s another part of the nation’s aging infrastructure that gets little publicity — at least until a fiery explosion destroys buildings or kills people. That infrastructure is the network of tens of thousands of miles of underground natural gas transmission lines running through most parts of the country and under the streets in most cities.

An investigation by USAToday last month found that since 2004, explosions and fires caused by gas leaks have killed 135 people, injured 600 and caused about $2 billion in damage.

The potential for dangerous explosions has grown as aging cast iron or bare metal pipes fail, causing leaks. Utilities across the U.S. have been replacing the old metal pipe, some of which is 100 years old, with plastic pipe. But the process often means digging up streets and as a result it’s expensive, costing $1 million per mile or more.

These underground pipes carry natural gas to 67 million homes and 5 million businesses across the country. Nearly 90 percent of the cast-iron pipe is found in 10 states in the Northeast. But natural gas explosions caused by leaking pipes can happen anywhere, which was made clear in the 2010 explosion in San Bruno, Calif., that killed eight people and leveled 35 homes.

Aging pipes and gas leaks are a national problem, even if worse in some regions of the U.S.

In Pennsylvania, a Pittsburgh newspaper reported last month that 31,000 natural gas pipeline leaks occurred in 2013.

The leaks the newspaper reported were mostly in distribution lines running from utilities to homes and businesses. But there is another network of underground pipes that move gas from wells to processing plants, or from one part of the country to another where demand is higher.

Only a few of this state’s 31,000 leaks in gas distribution lines did not cause fires or explosions. The escaping gas usually just leaks into the atmosphere. But as the ongoing debate over fracking has revealed, the methane in unburned natural gas is a potent greenhouse gas. Some critics of fracking argue that while gas is cleaner to burn than coal, the leaking methane tied to fracking and moving gas from wells contributes more to global warming that coal-burning power plants.

Beyond noting 31,000 gas leaks in a year, the newspaper’s report revealed another troubling issue — the locations of the leaks and the network of underground pipes is kept secret. The state Public Utility Commission and the courts back up that secrecy, saying the location of natural gas pipelines is a national security issue because the pipelines could be terrorist targets. Many people might see that as a convenient way to avoid transparency.

Beyond the legal issues, modern technology might shine more light on the gas leak issue and increase pressure on utilities to quickly repair the leaks and upgrade pipes. Last year, a Stanford University professor’s research had methane detectors attached to cars that were driven all over Washington, D.C., where the detectors found 10 times more leaks than the local utility, Washington Gas Light Co., had reported.

In other cities, a partnership between Google and the Environmental Defense Fund is putting methane detectors on cars that Google uses for the “street view” part of Google maps. The gas leak mapping project has covered Boston, Indianapolis and New York. More cities are set to have their gas leaks mapped.

In this region, Peoples TWP, unlike many other gas utilities, seems to welcome transparency about gas leaks. The company reportedly has asked for the EDF-Google leak-detecting project to come to Pittsburgh. A company spokesman said, “the more the public knows, the better.” That’s a refreshing attitude not often heard from other utilities.

All these developments will help focus national attention on the dangers posed by older gas lines as well as the climate change implications of leaks in transmission lines and throughout the gas-production process.

More national attention will be a good thing if it speeds up pipe and valve replacement to reduce gas leaks to as close to zero as possible.