Earlier this week, the massive methane leak spewing from an underground natural gas storage facility in California's Aliso Canyon passed a symbolic milestone: its duration exceeded BP's 2010 Deepwater Horizon oil spill in the Gulf of Mexico.

Now, a growing number of environmentalists, engineers and industry watchdogs say the disaster on the outskirts of Los Angeles could happen elsewhere. There are more than 400 underground natural gas storage sites spread across 31 states, and, like Aliso Canyon, decades-old equipment is deteriorating at many of them.

There is little federal oversight for the storage of trillions of cubic feet of potentially explosive fuel that is also a potent greenhouse gas. More than 100 facilities like Aliso Canyon that are owned and operated by local utility companies are subject to a patchwork of state regulations, which vary significantly from state to state. In California, multiple agencies have some responsibility for underground gas storage, with no designated lead authority, according to state lawmakers.

More than 200 other storage sites are part of an interstate natural gas pipeline network and fall under the jurisdiction of U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration (PHMSA). That agency, however, does not inspect or regulate these storage units, deferring instead to state regulators. After a storage leak in Kansas led to a fatal explosion in 2001, a federal court barred Kansas regulators from imposing tougher standards, leaving the state's storage facilities without any regulatory oversight.

"The issue of gas storage facilities is a classic example of an important piece of the natural gas supply chain that has just fallen through the cracks," said Mark Brownstein, a vice president in the Climate and Energy Program of the Environmental Defense Fund.

The inconsistent and patchwork regulatory structure, together with an aging and often repurposed infrastructure around the storage sites, could result in similar catastrophic leaks at other locations, according to Anthony Ingraffea, an engineering professor at Cornell University in Ithaca, N.Y., who is a leading researcher on methane emissions from the oil and gas industry.

"Aliso Canyon is not the only incident that has occurred—it's the most serious by far—but it's certainly not going to be the last," Ingraffea said.

Since the leak was first detected Oct. 23, an estimated 88,000 metric tons of methane, the greenhouse gas equivalent to the burning of 830 million gallons of gasoline , has blown into the atmosphere from a ruptured well that taps the underground Aliso Canyon formation where natural gas is stored.

California Gov. Jerry Brown declared a state of emergency. Thousands of residents from the northwest LA neighborhood of Porter Ranch evacuated their homes. Schools in the community closed after residents complained of nosebleeds, rashes, headaches and nausea. The Federal Aviation Administration declared a no-fly zone over the blowout, and residents are now calling on President Barack Obama to declare Los Angeles County a federal disaster area.

The well is part of one of the largest underground gas storage units in the country, holding more than half a cubic mile of natural gas roughly a mile and a half beneath the surface. Southern California Gas Co., which owns and operates the facility, hired outside well control specialists who have tried unsuccessfully to plug the blowout from above. A relief well they are now drilling likely won't be completed until late February.

The Aliso Canyon facility is one of 418 underground gas storage units nationwide, according to the Energy Information Agency. The biggest, with nearly twice the capacity of Aliso Canyon, is in Montana. Michigan, Illinois, Texas, Pennsylvania and Louisiana all store more natural gas underground than California. Nationwide, storage capacity has grown steadily over the past decade as hydraulic fracturing or fracking has created a boom in gas production.

The industry most often uses depleted oil and gas reservoirs, such as the one in Aliso Canyon, to stockpile the fuel near major markets to meet fluctuating demand.

The potential for catastrophic events because of leakage from these storage sites is well known, according to Gas Migration , a book published in 2000 by oil and gas engineers. Other scientists have compared the underground storage of methane, which is lighter than air, to "building a room for a bunch of feral cats all trying to escape," according to online magazine The Tyee.

And yet, "when it comes to regulatory attention, they are sort of the step-child," the EDF's Brownstein said.

A Regulatory Patchwork

The federal Environmental Protection Agency has authority under the Safe Drinking Water Act of 1974 to regulate the underground injection of hazardous and non-hazardous fluids, but an amendment to the act excludes natural gas storage facilities.

"We have minimal oversight of those types of facilities—in fact, we don't have any," Gina McCarthy, head of the Environmental Protection Agency, said in a Jan. 7 talk before the Council on Foreign Relations, a Washington think tank.

PHMSA has jurisdiction over approximately 233 of the 418 underground storage facilities that are considered part of the interstate gas pipeline network, but federal regulators deferred to state agencies in a 1997 advisory bulletin .

That document, issued by the Research and Special Programs Administration (RSPA), a precursor to the PHMSA, recommended state regulation and "voluntary industry action." The decision partly reflected industry input and guidelines for underground storage facilities developed by trade groups including the American Gas Association, the National Gas Supply Association, the Gas Research Institute and the American Petroleum Institute.

"RSPA, recognizing the value of underground hydrocarbon storage requirements tailored to a state's particular circumstances, is encouraging state action and voluntary industry action as a way to assure underground storage safety instead of proposing additional federal regulations," the bulletin concluded.

"The memo was a decision to just simply leave the field, and the Aliso Canyon incidence points to the fact that that was a bad choice," said Brownstein.

In California, state regulation of underground gas storage and the response to catastrophic leaks such as the one at Aliso Canyon is divided among multiple authorities.

"There currently is an alphabet soup of at least seven different state agencies that have roles in the response to a disaster of this nature," according to a bipartisan group of California state legislators who called for the designation of a single government agency to lead the state's response to the leak.

On Jan. 15, as the site continued to blow methane into the air, California authorities called for emergency regulations that would increase inspections and safety protocols to include daily leak inspections with infrared cameras to catch leaks at each of the state's 14 underground gas storage facilities.

Regulations vary from state to state. In Ohio, state regulators inspect storage wells once a year, employing visual methods only, a practice unlikely to detect leaks of an invisible and often odorless gas, according to reporting by the Columbus Dispatch.

The Ohio Department of Natural Resources, which regulates the state's wells and storage fields, has no records of any leaks, according to the paper.

Aging Pipes Create Hazards

The weak links in underground storage units of this type are the wells that go from the surface to the old oil and gas formations, according to Cornell's Ingraffea. These pipe- and cement-lined holes in the ground are used to inject gas into the reservoirs and extract it, often under high pressure.

Many of these decades-old wells were intended solely to extract the oil and gas originally in the formations, Ingraffea said. At Aliso Canyon, the well that failed is 63 years old. It is one of more than 100 wells at the site and one of tens of thousands of similar wells at storage units across the country, he said.

The problem has long been known to oil and gas engineers.

"Underground gas storage facilities and oil fields have demonstrated a long history of environmental gas migration problems," according to the book Gas Migration. "Experience has shown that the migration of gas to the surface creates a serious potential risk of explosion, fires, noxious odors, and carcinogenic chemical emissions."

The expected life of a gas storage facility is about 50 years, according to the book. The authors advise against locating an underground gas storage facility near an urban setting "because of the potential danger of gas migration to the surface, which can create an explosion hazard."

"The important question is not 'if' the storage facility will leak, but rather 'when,'" the authors write. "It is virtually impossible to ensure that the gas will not migrate to the surface."

The ongoing emissions from the California site have led to a growing chorus of environmental groups and government watchdog organizations calling for greater federal oversight.

"This crisis in California illustrates a huge problem with the lack of adequate regulation of underground energy infrastructure," said Tyson Slocum, director of the energy program for Public Citizen, a government watchdog based in Washington, DC. The Porter Ranch case is similar to that of the 2013 rupture of ExxonMobil's Pegasus oil pipeline in Mayflower, Ark., he said. Residents complained that no one told them a major oil pipeline ran underneath their community.

PHMSA does regulate the above-ground infrastructure of underground storage facilities that are part of the interstate natural gas network but doesn't touch the underground portions, which are subject to state regulations.

The one exception to this rule is Kansas. In 2001, a leak from a gas storage facility traveled 7 miles underground before surfacing in Hutchinson, Kan., where it caused an explosion that destroyed half a city block and killed two people. In August 2004, another leak at an underground gas reservoir 40 miles northeast of Houston shot flames 1,000 feet into the air and forced residents living within three miles to evacuate.

After the incident in Kansas, state regulators moved to toughen regulations for underground gas storage. Colorado Interstate Gas Co. (which didn't own or operate the reservoir that leaked) sued in federal court, arguing that jurisdiction over interstate facilities belonged to federal regulators. A U.S. district court ruled in favor of the gas company, barring the state from regulating underground interstate storage facilities in Kansas.

"As result of the court case, no regulator has inspected Colorado Interstate, or presumably any other interstate facilities in Kansas, for over five years," said Scott Anderson senior policy director for EDF's U.S. climate and energy program. "The company managed to get the court to rule that the state was pre-empted, even though the feds have no rules. It is obviously a risky situation to have neither state nor federal regulators regulating an activity that has led to explosions and deaths."

A bill introduced in the U.S. Senate in November by Sen. Deb Fischer (R-Neb.) would require PHMSA to regulate all of the country's natural gas storage facilities. For some, the legislation can't come soon enough.

"I think most people would be amazed that we didn't have any [federal] regulations," said Carl Weimer, executive director of the Pipeline Safety Trust, a nonprofit watchdog organization in Bellingham, Wash. "Certainly this huge leak in California now makes it clear that there needs to be some kind of minimum standards."