John Kelly

USA TODAY

About every other day over the past decade, a gas leak in the United States has destroyed property, hurt someone or killed someone, a USA TODAY Network investigation finds. The most destructive blasts have killed at least 135 people, injured 600 and caused $2 billion in damages since 2004.

The death toll includes:

• The explosion that leveled part of a New York City block in East Harlem in March, killing eight and injuring 48 more.

• A blast that flattened the concrete floors of an apartment building in Birmingham, Ala., killing one woman in December.

• A flash fireball in 2012 that left an Austin man dead, a scarred foundation where his house once stood and debris strewn across yards of his neighbors.

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The gas leaks that fueled those blasts are not uncommon. Neither is the cast-iron pipe — some of it more than a century old — that is the chief suspect in each of those three explosions and many others, according to the investigation by USA TODAY and affiliated newspapers and TV stations across the country.

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And those totals don't include tens of thousands more hazardous gas leaks that were caught before disaster struck.

The causes are many and complex, and often outside of the utility company's control, from construction workers hitting a gas pipe while digging to weather. But one nagging concern persists: aging bare-metal gas pipes that are susceptible to rust and corrosion, which can lead to leaks.

A review of federal data shows there are tens of thousands of miles of cast-iron and bare-steel gas mains lurking beneath American cities and towns — despite these pipes being a longtime target of National Transportation Safety Board accident investigators, government regulators and safety advocates.

VIDEO:Watch how cast iron pipes can lead to dangerous leaks

The Pipeline and Hazardous Materials Safety Administration has been pushing gas utilities for more than a decade to replace aging pipes with more resilient materials like plastic, though it's not required by law. The industry has responded by replacing thousands of miles of pipe, but a daunting amount remains. It can cost $1 million per mile, or more, to replace aging pipe, costs typically passed to customers.

Aging pipes are a high-risk example of the nation's struggle to replace its crumbling infrastructure, a danger hidden beneath the ground until a pipe fails or is struck by something and a spark ignites a monstrous blast. Natural gas is piped into 67 million homes and at least 5 million businesses, schools and other buildings across the country, with gas distribution and service lines snaking beneath most neighborhoods in American cities.

Leaking gas accumulating in buildings, basements and crawl spaces — if ignited — can explode with a force akin to a small but concentrated earthquake. The rapidly expanding pressure can blow out windows, roofs and support beams, collapsing buildings in an instant.

The review of federal databases and thousands of pages of government and utility company records has found:

• At least 85,000 miles of aging cast-iron and bare-steel gas pipes still operating in U.S. communities, despite decades of warnings from the NTSB, other investigators and federal safety officials that they're prone to failure and need replacing.

• Cast-iron mains, which can be 100 or more years old, are involved in a disproportionate share of significant gas leaks. About 2.5% of gas mains — the 2- to 24-inch pipes that deliver gas to neighborhoods and downtowns — are made of cast iron. But, the cast-iron mains are involved in about 10% of the most severe leaks from distribution pipes over the past decade, and about 12% of the fatalities.

• While the vast majority of the nation's 1,000-plus gas utilities operate very little or none of the most vulnerable cast-iron mains, the largest share is concentrated in heavily populated areas with a higher risk of catastrophic consequences. About 83% of cast-iron mains are in 10 states, mostly in the Northeast. More than half the iron pipe is operated by just 10 utilities. And one-third of it is buried in and around New York, Boston and Detroit.

• The 10 local gas utilities operating the most cast-iron mains also show up as above the national average — often far above – in several measures studied by USA TODAY. Most worrisome: Nine of the 10 utilities far exceeded the average rate for hazardous gas leaks in 2013. Three gas utilities, serving the New York and Philadelphia regions, reported hazardous leak rates more than 10 times the national average.

• Those utilities have a longer road to replacing all of their aging mains than the country as a whole. Pipes in crowded cities are harder to retrofit because of the cost and disruption to everyday life.

"The chance of a pipeline failing in any one place is pretty small, but if you live in one of those older East Coast cities with hundreds of miles of cast-iron pipe, that's just a failure waiting to happen," said Carl Weimer, the executive director of the Pipeline Safety Trust, a public-interest group advocating for more aggressive monitoring of the older pipe until it can be replaced.

Consider New York. The two companies that serve the New York City region — Consolidated Edison and National Grid — are above the national average in a wide range of measures.

In New York City and suburban Westchester County, the two gas companies reported about 8,400 hazardous gas leaks on their mains in 2013. More than half of the region's pipes were installed before 1970. And, about half of the gas mains are made from cast iron or unprotected steel — about seven times the national average of 7% for those vulnerable materials.

Con Ed's plans call for replacing about 65 miles of cast-iron and bare-steel gas mains a year at a cost of $215 million, said company spokesman Allan Drury. Replacing all of the company's cast-iron and bare-steel pipes would cost $10 billion across a network that includes Manhattan and Westchester County, the company said. In the meantime, Con Ed is increasing inspections, tests and other checks on its system.

The company's focus — mirroring other utilities — is based on more factors than just age.

"Con Edison uses a risk-based approach to prioritize main replacement," Drury said. "We replace main sections by priority based on a number of factors including pipe diameter, pressure, prior leak history and proximity to buildings, material, soil conditions, age and other conditions. Old does not mean bad; some newer iron pipes may merit replacement because of their location in corrosive soil."

Reports of gas leaks from the public have spiked, too, since the East Harlem incident, and utilities respond to those reports by aggressively hunting for piping trouble spots, utility officials said.

In the nation's capital, 62% of gas mains are at least a half-century old. About 4% of Washington Gas & Light's natural gas went unaccounted for in each of the past two years, according to its annual reports to federal regulators. Nationally, the average amount of "unaccounted for" gas reported by utilities was just over 2% in 2013, utility reports show. Experts say the missing amount can include leaked gas, but also measurement mistakes and other issues.

"We have approved an accelerated program where more gas mains, particularly the cast-iron ones and the bare-steel ones, will be replaced on a little faster schedule," said Betty Ann Kane, chairman of DC Public Service Commission that oversees the private utility Washington Gas. "The system is safe, believe me, we have inspections going on all the time."

The dilemma isn't isolated to big cities.

In Pensacola, Fla., the share of cast-iron and bare-steel pipe is four times the U.S. average. Pensacola Energy couldn't account for 4.5% of its gas, its 2013 report says.

The Gulf Coast community plans to replace 20 miles of old pipe a year, at a price tag of $2.4 million a year. At that pace, the work will be done in 2067.

Whether that's fast enough is unclear, said Paul Oleksa of Akron, Ohio, a veteran pipe inspector. "If pipe has been in the ground, for, say, 50 years, and it's done a great job, fine. But the probability of a leak gets greater as it gets older, so it's prudent to replace it. How quickly? There's no hard and fast answer."

In dozens of cities examined by USA TODAY and affiliated journalists, gas company and state records show that replacing old pipes will take 10 to 20 years in some communities, but stretch out beyond a half-century in others.

Asked if that's fast enough, Cynthia Quarterman, director of the federal Pipeline and Hazardous Materials Safety Administration, said, "Personally, no, not for me."

Quarterman said that while there has been progress replacing pipe in many states, her agency will continue using its bully pulpit to pressure utilities and state agencies to move faster. For instance, every year, her agency posts updated figures showing how much troublesome pipe remains in the systems of every utility and state.

The agency also enforces a federal law requiring gas utilities to file Distribution Integrity Management Plans with state pipeline regulators, outlining the biggest threats to their pipe networks and what they're doing to make sure gas leaks don't threaten people or property.

However, PHMSA doesn't have the authority to require replacement by a specific date. "That's a question Congress will have to answer," she said.

To be sure, the gas utility industry is making progress getting aging pipe out of the ground after more than two decades of warnings from the NTSB, other regulatory agencies and safety advocates.

The utilities replaced about 10,000 miles of the most vulnerable cast-iron pipe from 2004 through 2013, leaving 30,000 miles to go. They replaced an additional 17,000 miles of bare-steel mains during the same period, leaving about 56,000 miles still in operation.

The government has worked with the industry to pass laws in 38 states making it easier for utilities to speed up replacement, orchestrating new ways for utilities to raise customers' rates to pay for the new pipes.

"We have a strong safety record. We are not an industry that rests on its laurels," said Lori Traweek, senior vice president of the American Gas Association, a trade group that represents gas utilities across the country. "Every incident is one that you want to look closely at to see how it could have been prevented."

There is no one-size-fits-all approach, according to AGA. The group questions drawing conclusions about the safety of a pipe based solely on what it's made out of, saying that even systems in big cities with high concentrations of cast-iron and bare-steel gas mains can be operated safely if the utility has in place aggressive inspection, monitoring and mitigation programs.

The group's leaders also say it's important to note the government has not required the replacement of cast iron and bare steel. Rather, the government is encouraging replacement together with increased assessment of the overall integrity of an area's pipes and flexibility to focus on the biggest threats. For instance, Con Ed spends $500 million a year on improving its infrastructure in New York each year — not just the $215 million on pipe replacement.

Replacement is a challenge because of both cost and logistics. To unearth, remove and replace an old cast-iron gas main can cost more than $1 million a mile. It means cutting into streets, navigating a rat's nest of other pipes and underground infrastructure, shutting down roads and sidewalks. The cost can be several million dollars a mile in major cities, where those disruptions are amplified by the presence of more people, buildings and cars.

"There are so many different issues with replacing that (old) pipe," Traweek said. It's not just age that is most important, or even material. "What's most important is the integrity of the pipe is being managed, so that the utility knows what they need to address first."

Richard May of Austin, whose neighbor Renald Ferrovecchio, 43, died when gas leaking from a cast-iron main exploded and leveled his house in 2012, was surprised to learn of the long-standing warnings about cast-iron gas pipes. He and his neighbors pressed for replacement of the remaining old pipes in their area.

"The ranges of emotions go from angry to frightened," May said. "There's a certain trust that we consumers have, that we Americans have and, the infrastructure and the energy that we rely on. And to have your neighbor killed right there and then hear about all these potential threats, it's unsettling. It's frightening. It boggles my mind."

Contributing: Kala Kachmar, Montgomery (Ala.) Advertiser; Elizabeth Ganga, The (Westchester County, N.Y.) Journal News; Terri Gruca, KVUE-TV in Austin; Russ Ptacek and Erin Van der Bellen, WUSA-TV, in Washington; Russ Walker and Linda Byron, KING-TV, in Seattle; Rob Johnson, Pensacola (Fla.) News-Journal; Lauren McLendon, USA TODAY;