The Navy is currently cleaning up a fuel leak at its Red Hill Bulk Fuel Storage Facility that occurred late Thursday night and has begun investigating how it happened.

“Navy personnel responded to and contained a reported fuel release, initially assessed at approximately 1,000 gallons,” Navy Region Hawaii confirmed in an e-mail. “As designed, the fuel release went into a containment system in the tunnel where the pipeline is located, and the fuel was recovered.”

So far Navy officials say they don’t believe any fuel has leaked into the environment outside of the facility. The Navy’s aging tanks at Red Hill have long been a point of concern for local residents and organizations that worry the massive underground fuel storage facility is a threat to Oahu’s drinking water.

“They have a long history of leaking. And the Navy itself said there was a 27.6% chance of the tanks leaking up to 30,000 gallons of fuel every single year,” said David Kimo Frankel, an attorney for the Sierra Club of Hawaii in a press release from the organization. “It’s only going to get worse from here. The tanks need to be drained.”

Studies over the years have detected petroleum contamination in the groundwater beneath the tanks.

The last reported instance was in 2014, when according to the EPA 27,000 gallons of fuel leaked from the facility. Tests of the water in wells surrounding the tanks showed a spike in levels of hydrocarbons in soil vapor and groundwater, but test results also ultimately concluded it was still within safe levels.

The Navy’s facility at Red Hill is unlike other fuel storage stations. Completed in 1943, the Navy placed the tanks underground beneath a volcanic mountain ridge to make them harder for Japanese military forces to strike. One of the largest fuel storage facilities in the country, it can store up to 250 million gallons of fuel. In 1995 the American Society of Civil Engineers declared it a landmark.

But the tanks’ age, size, location and proximity to Oahu’s aquifer have provided unique challenges for maintenance. Since 2006 the Navy has spent over $260 million on efforts to upgrade the facility. The Navy has been working with the University of Hawaii to look for ways to modernize the tanks and in October proposed inventing a way to achieve “double wall equivalency” – essentially a tank within a tank.

When the Navy officially submitted its latest plan for review, both the Environmental Protection Agency and Hawaii Department of Health rejected the plan calling it “deficient,” and asked the Navy to revise and resubmit its plan. The Sierra Club and other local organizations have criticized the plan, arguing the Navy is opting for the cheapest fix rather than the most effective.

“This latest leak proves, once again, that the 78-year-old Red Hill fuel tanks are deteriorating and pose a serious threat to drinking water for hundreds of thousands of Oahu residents. It’s time to retire the tanks,” said Kyle Kajihiro of Hawaii Peace and Justice.

Red Hill fuels the military’s ships and aircraft as they conduct operations across the Pacific. The military has been conducting nearly constant training and operations in the region amid increasing tensions with the Chinese military. The Navy has also argued the tanks are critical as a backup for planes and ships in the event that a natural disaster damaged fuel storage at Honolulu’s airport or harbor.

U.S. Sen. Mazie Hirono said in a Friday press release that she called Adm. John Aquilino, the top officer at U.S. Indo-Pacific Command at Camp Smith and the most senior military official in the region. Aquilino previously headed the Hawaii-based U.S. Pacific Fleet before taking on his current post last month.

“While the initial assessment is ongoing, Adm. Aquilino assured me that the detection and response system worked and the fuel release is contained,” said Hirono.

This year Hirono became the chair of the Senate Armed Services Subcommittee on Seapower, making her one of the most influential figures in the federal government when it comes to the Navy’s funding and procurement policies.

“I will continue to closely monitor and engage with military leadership and the local community regarding the Red Hill facility,” she said. “This incident reaffirms the need for clear and transparent information to be communicated in a timely manner.”

The most recent iteration of Congress’s annual defense funding bill included a provision requiring the Navy to review available technology for upgrading the tanks every five years.

“Our containment system functioned as designed to keep the fuel contained within our facility, with no indication that fuel was released to the environment,” said Capt. Gordie Meyer, the head of Naval Facilities Engineering Command Hawaii in a press release. “We are conducting an extensive investigation to determine the cause of the release and will continue to examine our systems and procedures.”