In the wake of a fiery explosion that ripped through a San Bruno, Calif., subdivision, it might be natural to reassure ourselves that the incident was a shocking and tragic unlikelihood—natural, but wrong.

Experts say the Sept. 9 gas main blast that killed seven people and damaged nearly 60 homes was more than a fluke. It was an accident waiting to happen.

"In reality, there is a major pipeline incident every other day in this country," Pipeline Safety Trust Executive Director Carl Weimer told the Associated Press. "Luckily, most of them don't happen in populated areas, but you still see too many failures to think something like this wasn't going to happen sooner or later."

Unfortunately, an aging infrastructure of decaying gas lines, coupled with a booming population and lax oversight, means what happened in San Bruno could happen almost anywhere.

"If this was the FAA and air travel we were talking about, I wouldn't get on a plane," says Rick Kessler, also of the Pipeline Safety Trust, according to the AP.

Although the National Transportation Safety Board is investigating the San Bruno explosion—its final report is expected late next year, according to the Los Angeles Times—there is speculation that the explosion could have resulted from one or several factors, including corrosion of the aging pipe or damage caused by nearby sewer work. The loss of life and property is thought to have been exacerbated by Pacific Gas and Electric's inability to shut off fuel to the line and population growth in the area since the pipe was installed in 1956.

"That's an issue we're going to have to look on a bigger scale—situations in which pipes of some age were put in before the dense population arrived and now the dense population is right over the pipe," NTSB Vice Chairman Christopher Hart told the AP.

Of the roughly 300,000 miles of gas transmission lines in United States, only 7 percent are highly populated, or what the gas industry terms “high consequence,” according to the AP article.

Still, those lines are part of a nationwide web of roughly 2 million miles of gas and oil pipeline, some as which are many as 120 years old. Although, the U.S. Pipeline and Hazardous Materials Safety Administration is responsible for regulating the repair and upkeep of the lines, the inspector to pipe ratio is overwhelming, Weimer told the AP.

"When you look at two-and-a-half million miles of pipeline with 100 inspectors, it's not reassuring," he said. "To a grand degree the industry inspects and polices themselves." That method doesn’t always work. According to the article, there have been “2,840 significant gas pipeline accidents since 1990, more than a third causing deaths and significant injuries.”

Even though the gas industry says it spends millions testing and replacing infrastructure, there are problems prioritizing replacement with such vast needs. In the case of the San Bruno explosion, a nearby section of pipe was identified as an “unacceptably high” risk in 2009, but the $5 million in PG&E ratepayer money approved to replace it was allocated to more pressing issues, according to another Los Angeles Times article.

"We constantly monitor our system and if at any time we identify a threat to public safety we act to repair it," PG&E Spokesman Andrew Souvall told the Times. "We always take a proactive approach toward the maintenance of the lines."

Even so, another PG&E spokesperson said that about a quarter—1,021 miles—of the company’s high consequence gas lines hadn’t been inspected yet. And in cases such as the San Bruno pipe, technology used to inspect for leaks can’t be employed because of outdated configurations.

The accident has set off a flurry of investigations and legislation aimed both directly at PG&E and at industry-wide safety. In addition to a mandate from the California Public Utilities Commission that PG&E examine its system statewide, there have been calls for federal examination of the lines, requirements for shut off valves, and increased fines for safety violators. California Senator Alex Padilla said the state needed to ramp up regulation and enforcement, according to the Times.

"This horrible accident is a wake-up call that California needs to do more to protect the public and meet the highest safety standards," he said.