Security concerns about the South China Sea often focus on the ships that traverse it, including in so-called freedom of navigation operations run by the U.S. Navy and recent efforts by Chinese fishermen and coast guard units to take control of the lucrative fishing business in the region. But another element of maritime security has received less attention: submarines. The Navy’s “Silent Service” rarely discloses its operations, but is part of a diverse and growing international fleet of submersibles that is deployed across the Pacific region broadly and in the South China Sea specifically.

Adm. Scott Swift, the top officer of the U.S. Pacific Fleet, said in an interview that submarines are a “critically valuable asset” to him. Surface-to-air missiles and other weapons in the region are deployed as part of a concept known as anti-access area denial (A2AD) to hinder the movement of adversaries, but submarines aren’t affected by them like surface ships and aircraft because they’re below the surface, he said.

“It gives me much more open access to areas that would be more contested in a conflict for surface units, for instance, or air units, potentially,” Swift said.

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The United States, China, India, Vietnam, Indonesia, Malaysia and Australia are all among the countries looking for ways to upgrade submarine operations in coming years. Swift said it’s in part a “reflection of the angst” in the theater with respect to not just China, but others as well.

“It’s certainly centralized in the minds of many in the South China Sea, but we see it more broadly and certainly in the East China Sea and elsewhere,” Swift said.

The Pentagon expects to spend about $97 billion alone in coming years on what it calls the Ohio-class replacement program, phasing out 14 existing nuclear missile submarines with a new generation of vessels that includes 12 more. Additionally, it has been buying a new generation of attack submarine called the Virginia class since 1998, phasing out old attack subs in its Los Angeles and Seawolf classes while also investigating how it can expand operations with unmanned submarines.

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“We’re investing over $8 billion just next year to ensure ours is the most lethal and most advanced undersea and anti-submarine force in the world,” Carter said last week while previewing his trip to Asia at the Council on Foreign Relations in New York. “That includes new undersea drones in multiple sizes and diverse payloads that can, importantly, operate in shallow water, where manned submarines can’t.”

Some national security analysts have speculated that part of China’s desire to take over all or part of the South China Sea is to create a sanctuary for its submarines. It includes some areas that are more than 1.5 miles beneath the surface, and underwater canyons where a submersible could hide.

In December, China deployed a new nuclear-powered ballistic missile submarine, the JIN-class, adding a vessel capable of carrying sea-based nuclear missiles for the first time. China also deployed attack submarines to the Indian Ocean for the first time in 2014, ostensibly to support counter-piracy operations but more realistically to gain familiarity with the region and to demonstrate an emerging capability, according to the Pentagon’s annual report on China military operations released last year.

Marine Lt. Gen. Vincent Stewart, the director of the Defense Intelligence Agency, predicted in testimony before the Senate Armed Services Committee in March that China’s People’s Liberation Army (PLA) is likely to continue adding more military equipment in the South China Sea, something that could again increase China’s A2AD abilities in the region.

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Stewart said China’s vocal opposition to freedom of navigation operations by the U.S. Navy in the South China Sea demonstrates “that Beijing recognizes the need to defend these outposts and is prepared to respond to any military operations near them.”

Freedom of navigation operations occur when the Navy sends a ship, usually a destroyer, through a region in which a country such as China has made maritime claims that the United States considers excessive. The Navy ran them through the South China Sea with the USS Lassen in October and the USS Curtis Wilbur in January, prompting allegations from Beijing that the United States violated Chinese law by entering what it considers its territorial seas.