About 240,000 miles of pipelines ferry oil and natural gas from wells to refineries across the United States. And amid a boom in natural gas, the network continues to grow.

State and federal regulators, however, know little about each privately built “gathering” pipeline, and in some cases, may not even know where they lie. As a result, many of the pipelines are not inspected for damage, according to an audit released recently by the U.S. Government Accountability Office.

The use of pipelines is seen largely as the safest way to carry oil and natural gas, killing hundreds fewer people each year when compared to transport by freight trucks or trains. Pipeline transport, however, still can prove dangerous. Corrosion or other pipe damages can lead to leaks, ruptures and, in rare cases, explosions.

The U.S. Pipeline and Hazardous Materials Safety Administration regulates just 10 percent of natural gas and oil gathering pipelines, according to the federal audit.

For most of the unregulated lines — spanning some 200,000 miles mostly through Alaska, California, Louisiana and Oklahoma — regulators leave inspections and maintenance to private operators, who are not required under federal law to report the data. That void in information could hide underground dangers, state regulators say.

“Gathering line operations may affect the public and should be held accountable,” wrote Paul Metro, a Pennsylvania regulator and chairman of the National Association of Pipeline Safety Representatives, in a letter last August asking federal agencies to adopt stricter standards for safety and transparency.

“All pipelines should be treated the same in terms of safety from the well head to the city gate.”

Most unregulated pipelines operate at lower pressures and away from population centers, but they still present safety risks, state regulators say, particularly as increased shale drilling sparks development across pipeline rights of way.

New methods of oil and gas extraction also have led companies to develop wider, higher-pressured pipelines that are as powerful as some regulated lines but still escape federal oversight.

That’s the case for some gathering lines in Fort Worth, Texas, according to a 2010 report prepared for the city’s league of neighborhoods. The growing city is the country’s 16th-largest in population. Just four state agencies collect any data on unregulated oil pipelines, according to the GAO audit, while seven states do so for gas lines.

In Pennsylvania, where the Marcellus Formation has brought a years-long shale oil drilling boom, state regulators had long lacked such information about natural gas pipelines. In fact, Pennsylvania was one of just two oil and gas-producing states that had no oversight at all over pipelines.

That was until late 2011, when Gov. Tom Corbett signed legislation giving the Pennsylvania Utilities Commission power to enforce federal safety laws and to require data disclosure.

Still, the law doesn’t apply to pipelines in the state’s most rural areas. “It’s an evolving process,” says Jennifer Kocher, a spokesperson for the utilities commission.

The GAO report says the federal government should create an online clearinghouse to ease data sharing among strapped state agencies. The federal pipeline agency is considering rules that would ramp up data collection on gathering pipelines, but the proposal, which faces pushback from the oil and gas industry, remains in its early stages.

Jim Malewitz is a Stateline.org staff writer. Email at [email protected]. This article is reprinted from Stateline.org, a nonpartisan, nonprofit news service of the Pew Center on the States that reports and analyzes trends in state policy.