In 2010, when rubberlike quieting material started to peel off the hulls of newer Virginia-class submarines, the Navy said it was fine-tuning a fix for a problem occurring on the first few ships made.

Seven years later, the Navy still appears to be seeking a cure.

When the $2 billion USS Mississippi recently returned to Pearl Harbor, its “Mold-In-Place/Special Hull Treatment” looked ragged and was missing chunks on at least one side of the hull. The sub was commissioned in 2012.

The loss of stealth comes at a time when China and Russia are making worrisome advances in submarine technology.

A photo that appeared on Facebook prompted the comment that the Mississippi looked “pretty banged up.” No collision, no accident, and no hull damage, reported the Pacific Fleet Submarine Force at Pearl Harbor.

The Naval Sea Systems Command in Washington skirted questions about what happened to the Mississippi and how much of a problem the debonding remains for Virginia-class attack boats.

Asked what caused the damage, the command in an email cited the “wear and tear from the harsh environment in which the submarine operates,” but would not say when or why it occurred.

The Honolulu Star-Advertiser also asked how much of a problem debonding remains across the Virginia-class fleet, given past problems with the hull treatment that is applied in sections.

“Navy and industry continue to find efficiencies and improvements in the construction and maintenance of Virginia-class submarines,” the command said in the emailed response. “An integrated process team was assembled to address conditions such as those reflected in the (USS Mississippi) photograph, and improvements to materials, processes and testing were subsequently identified, evaluated and implemented. The Navy is continually assessing and developing more effective solutions.”

Bryan Clark, a senior fellow at the Center for Strategic and Budgetary Assessments and a former Navy submariner, said the amount of acoustic coating missing on the Mississippi “could create enough flow noise to be a sound problem at even relatively slow speeds. Also, there is enough tile missing that it could reduce the coating’s ability to absorb sonar energy and make the submarine easier to find with active sonar.”

Clark said it isn’t clear from the photo if the tiles came off due to debonding, meaning a loss of adhesion, “or if they got stripped off from something rubbing against the submarine. Nets and cables adrift at sea can do this.”

Cmdr. Corey Barker, a spokesman for the Pacific Fleet Submarine Force, said that in terms of possible abrasion, he was “not aware of anything of that nature” happening.

The 377-foot attack submarine returned to Pearl Harbor on Feb. 13 after being in and out of port for routine training, Barker said. On Sept. 1, the Mississippi came home after a six-month deployment to the Western Pacific.

A Navy photo taken June 13 during a port stop in Busan, South Korea, shows the same side of the sub missing a few coating pieces, but not anywhere close to the degree of loss exhibited last month.

Asked if the debonding occurred during the deployment, the Naval Sea Systems Command said, “As a matter of (Defense Department) policy, we do not discuss the specifics of submarine operations.”

Anechoic, or echo-reducing, tiles were used by Nazi Germany in World War II. The application helps break up incoming sound waves and reduces the sound that travels back from sonar. The Soviets adopted the use of the coating, and the U.S. Navy followed in 1988.

The USS Hawaii, Texas and North Carolina, all now based at Pearl Harbor and among the first Virginia subs to be built, were part of a group of about six of the vessels identified in 2010 as having a problem with the mold-in-place urethane coating.

“We’ve been made aware of the issues, we’re making improvements in the process, and we’re seeing results already,” the Associated Press quoted Alan Baribeau, a Naval Sea Systems Command spokesman, saying at the time.

The website Next Navy posted photos from 2013, however, showing the submarines Minnesota and Missouri with some coating coming off. The Navy has been replacing older Los Angeles-class subs at Pearl Harbor with the more capable Virginia-class. Four Virginia subs are based in Hawaii now. The USS Illinois is expected to be relocated to Oahu.

Naval analyst and author Norman Polmar, who served as a consultant to three secretaries of the Navy, said it’s a glue issue with the acoustic material.

“Remember, (the coating sheets) are external to a submarine, which is going from surface pressure down to, let’s say, 1,000 feet occasionally,” Polmar said. “In addition, the temperature changes radically.”

The glue has to “take the constant changes in pressure, constant changes in temperature, and it ain’t an easy thing to do,” he said. Additionally, submarines periodically brush against floating debris, against a pier, or “rarely, but sometimes, against another submarine.”

The Pearl Harbor Naval Shipyard said in 2015 that it was working on special hull treatment restoration on the USS Hawaii. Naval Sea Systems Command did not disclose when the Mississippi will receive repairs or the estimated cost.

Clark, with the Center for Strategic and Budgetary Assessments, said the Navy always has had difficulty keeping anechoic coatings on submarine hulls. Since the sound energy from sonar hits the whole submarine, a few missing tiles will not significantly affect the return, he said.

But he said the Navy will have to continue to improve the acoustic coating’s resilience with other countries starting to use more active variable-depth sonars on ships and helicopters that can be positioned to more effectively look for submarines.

The Russians have made significant strides in acoustic technology on the Severodvinsk-class submarines, and the U.S. Navy’s Acoustic Superiority Program is an attempt to stay ahead of the pack. The sub USS South Dakota, expected to be christened this summer, is being used as a test for an improved acoustic coating and noise-reducing machinery.

General Dynamics Electric Boat in 2012 said it was delivering the Mississippi to the Navy a year ahead of contract schedule and more than $60 million below target cost. The USS Mississippi Commissioning Committee reported the sub’s cost at $2 billion.

A 2016 Congressional Research Service report said the procurement cost of two Virginia-class subs in fiscal 2017 was $2.7 billion each.

That same year, Rear Adm. Charles Richard, director of undersea warfare, and Rear Adm. Michael Jabaley, program executive officer for submarines, told members of Congress that the need for submarines is only growing.

“As the threat from adversary advances in sensors and weapons such as cruise missiles, anti-ship ballistic missiles and integrated air defense systems grows,” the pair said in written testimony, “undersea forces will be increasingly asked to accomplish missions once conducted by forces that are now held at increased and potentially unacceptable risk by the improved range, precision, and lethality of advanced systems.”