Billions of dollars are needed to update the vast network of natural gas pipelines, some at least 90 years old, beneath Pennsylvania, a newspaper report found.

About 20 percent of the natural gas pipelines in Pennsylvania were installed before 1960, twice the average national age of pipeline infrastructure. Data compiled by the Pittsburgh Tribune-Review found a gas leak reported in one out of every three miles of pipe last year, making it one of the leakiest networks in the United States.

"We have a ticking time bomb under most of our cities, especially in the Northeast where we have older cities," Allentown Mayor Ed Pawlowski said in a report published Tuesday.

Gas explosions in the last 10 years left 10 people dead and 21 others injured, the Tribune found.

Pennsylvania is home to the Marcellus shale natural gas basin. Energy consultant group Wood Mackenzie estimates that, by 2020, Marcellus should produce an average of 20 billion cubic feet of equivalent per day, which it said would represent 25 percent of the total U.S. natural gas supply.

With that, the state's Public Utility Commission said it needs to spend about $11 billion to overhaul the state network of gas distribution lines.

"How do we pay for all of that?" Pennsylvania Gov. Tom Corbett told the newspaper. "Replacing [a pipeline] isn't as nice as building a park or building a bridge, is it?"