THE BURNED OVER DISTRICT

NOT! Nothing put into the ground can expect to last forever, not the least iron piping. But that is what so much of our underground utilities travel through. And as ConEd is regularly finding out, when pipes are older than 50 years, the likelihood of a leak is great, as is the likelihood of a catastrophe.

To replace all of the old mains in its network right now would cost as much as $10 billion, Con Edison estimates. Much of that expense would fall on the residents and businesses that use the gas for heating and cooking.

Despite the high cost and logistical hurdles, alarmed regulators at the state’s Public Service Commission have ordered the company to significantly step up its replacement schedule, from 50 miles of pipe a year to 70 by 2016, in the city and in Westchester. Even at that rate, it would still take nearly three decades for the utility to finish swapping out what regulators have identified as the most leak-prone pipes.

As a result, infrastructure experts say there could easily be more explosions like the one this month in East Harlem.

After the blast, federal investigators identified a leak in the gas main, but they are still not certain what caused it or if it was the source of the gas that exploded.

Federal records show the New York City utilities have been able to cut into their leak numbers as they have replaced mains. National Grid, in particular, has made improvements. Its rate of leaks per 100 miles of gas mains still ranks among the highest in the country, but it is significantly better than Con Edison’s.

Con Edison has made progress, too. But last year, when regulators were considering whether to let Con Edison raise its rates, the commission’s staff voiced concerns about the company’s attitude toward safety.

The staff testified that Con Edison had 695 violations of the state’s gas pipeline safety regulations over the previous three years. Not all of those violations were classified as “high risk,” but the regulatory staff said any failure to follow the rules was “a serious issue that could either directly or indirectly lead to an incident causing serious public harm.”

A spokesman for Con Edison, Michael Clendenin, responded by saying the company “takes compliance with the commission’s regulations very seriously.” He added that the complexity of New York City’s infrastructure probably accounts for the utility’s high rate of leaks, but added, “We attend to hazardous leaks immediately.”

In order to ignite, gas has to pool in a confined space until it makes up at least 5 percent of the air. Then, any flame or spark — even the flipping of a light switch — can set it off.

In the last decade, The Times identified from federal records 22 significant gas ignitions in the city; a dozen of these were categorized in federal records as full-fledged explosions.

Not counting the blast in East Harlem, gas-related episodes have killed three people in the city in the last decade and injured 22 others, according to a tally by The Times.

The East Harlem gas explosion, which also injured dozens, was the first fatal one in the city in nearly five years.

The new generation of plastic gas piping may last longer, but only if it is put inplace before the next leak.