A month after a fiery explosion claimed two lives and destroyed five rowhouses on a South Philadelphia block, officials said a crack in a 92-year-old natural gas main led to the blast.

Though an investigation into the root cause of the cracked main is ongoing, concerns over the city’s aging gas line infrastructure are well-established.

What caused the pipe to break?

What caused this specific gas main, constructed in 1928, to crack and leak natural gas is under investigation by the Pennsylvania Public Utility Commission.

Typically, a cast-iron pipe can break in two ways, said Joe Martin, a civil-engineering professor at Drexel University. A joint, made of lead and oakum, connecting two cast-iron pipes can weaken over time and break. Or, corrosion on the pipe can cause a crack, Martin said. Heavy vehicles driving on the street can create additional stress on the mains below.

The fundamental problem, Martin said, is that cast-iron gas pipes grow brittle with age, and many in Philadelphia are about a century old.

“The fact that they’re lasting 100 years is a wonder,” Martin said.

When will Philadelphia’s cast-iron pipes be replaced?

About half of Philadelphia Gas Works’ 3,000 miles of gas mains are cast iron. In 2016, the utility sped up efforts to replace its riskiest pipes, and replaced about 35 miles last year.

A PGW spokesperson said the utility aims to replace all of Philadelphia’s cast-iron mains by 2057.

The aging, fragile cast iron is being replaced by plastic PVC piping, which Martin said is “safer, faster [to replace], and lasts longer.”

You can find PGW’s current pipe replacement projects here.

How worried should residents be?

Martin described the state of Philadelphia’s gas mains as "definitely cause for concern.”

He said Philadelphia’s rowhouses are built with strong firewalls between them, but proximity and connected porches mean that if something happens in one house, adjacent neighbors are at risk.

“The bottom line,” Martin said, is that “this is the typical age of pipes in the city. And it’s too old.”

What should you do if you smell gas?

Call 911 or PGW’s emergency hotline at 215-235-1212.

Staff writer Andrew Maykuth contributed to this article.