CIPHER BRIEF REPORTING — The Federal Communications Commission (FCC) and members of Congress are warning that China may be engaged in underwater espionage – accessing government and private-sector data that travel via the vast global network of undersea cables.
The FCC voted Thursday to accelerate the deployment of American-made submarine cable systems, and prohibit the use of technology manufactured in China in any subsea cables that reach the United States. And last month the chairs of three House committees wrote to the CEOs of Google, Meta, Microsoft, and Amazon, asking them to identify the extent to which the subsea cable systems they use are produced, maintained, or repaired by China-based firms.
In their letter to the big tech companies, the members of Congress called undersea cables “one of the most strategically significant, and increasingly vulnerable, components of the world’s digital infrastructure…powering not only global commerce and innovation but also the core operational systems of national security, intelligence, and defense,” and they warned that the cables could “become a backdoor for espionage, disruption, or exploitation of U.S. data and communications assets.”
More broadly, the undersea cable questions are the latest in a series of concerns about actions taken by Beijing to infiltrate American critical infrastructure, following cyberattacks and breaches of U.S. water systems, power grids and other networks.
“The Chinese have been grabbing big data from all forms of communication that traverse the earth, including a substantial amount of U.S. and allied data,” Rear Admiral (Ret.) Mike Studeman, a former Commander of the Office of Naval Intelligence, told The Cipher Brief. “This is not some kind of theoretical threat. This is trying to stop something that's underway.”
The potential infiltration of the undersea cable network is “a significant threat,” Nick Thompson, a former CIA paramilitary officer and Naval Special Warfare Development Group operator, told The Cipher Brief. “China has invested heavily in cable repair infrastructure, and combined with its constant nefarious maritime activity throughout the world, it’s logical to elevate the risks to the highest levels of [the U.S.] government.”
Sabotage and espionage on the ocean floor
As The Cipher Brief has reported, undersea cables have become a vast and largely unseen piece of critical global infrastructure. Roughly 650 cables cover more than 800,000 miles of ocean floor, carrying a staggering 98 percent of the world’s data – everything from e-mail traffic to military communications to an estimated $22 trillion in financial transactions processed every day.
“When you think about the capacity of those cables, it's terabytes of information versus gigabytes of information through satellites,” Rear Adm. Studeman said. “When you go through satellites, it's like drinking a glass of water in terms of the amount of data throughput. But undersea cables, it's like trying to drink a large swimming pool worth of data. And so the threat is significant…people trying to get into your communications, manipulate them, or outright disrupt them through severing and cutting.”
The U.N. estimates that between 150-200 incidents of undersea cable damage occur each year, and while most are accidents involving dredging operations, dragged anchors, or natural disasters, cables have also been targeted by saboteurs, operating in what one report called the “gray zone of deniable attacks short of war.”
Russia and China have been accused of intentionally severing cables, particularly in the Baltic Sea and the waters near Taiwan. In one of the most widely-reported cases, Taiwan said that two submarine cables leading to its island of Matsu had been cut in 2023, causing widespread internet outages. Taiwan blamed two Chinese vessels for the damage, and officials in Taipei said they had documented 27 incidents since 2018 of Chinese vessels damaging undersea cables that served the island.
China and Russia have denied tampering with any undersea cables.
Sign up for the Cyber Initiatives Group Sunday newsletter, delivering expert-level insights on the cyber and tech stories of the day – directly to your inbox. Sign up for the CIG newsletter today.
The latest concerns are less about cutting cables, and more about the entities that manufacture and maintain the undersea network. More than 90 percent of the world’s subsea cables are manufactured and installed by four private firms: the American SubCom, France’s Alcatel Submarine Networks, Japan’s Nippon Electric Company and China’s HMN Technologies. According to a report by the Center for Strategic and International Studies (CSIS), HMN, formerly known as Huawei Marine Networks Co., Ltd., has been the world’s fastest-growing subsea cable builder over the past decade, and accounts for 18 % of the cables currently on the ocean floor.
HMN and the State-controlled Chinese firm S.B. Submarine Systems (SBSS) are also major players in the cable repair space – and they routinely underbid other companies for the repair work. According to CSIS, HMN Technologies’ bids for undersea cable projects are priced 20 to 30 percent lower than its rivals.
“If we rely on China for repairs, then something can be inserted in that process to tap that particular cable,” Beth Sanner, a former Deputy Director for National Intelligence at the ODNI, told The Cipher Brief. “Anytime a Chinese ship repair operation is happening…all of these companies report back to Beijing. So I consider that an absolutely high risk.”
Thompson noted that China offers an unparalleled suite of maintenance and repair services for the subsea cable networks – they have “available assets, they have the technical skill, and their services are much cheaper than Western companies,” he said. And the CSIS report warned of frequent repairs done by “high-risk vendors, some of whom are Chinese.” It found that “the overreliance on Chinese repair ships due to limited alternatives in the marketplace is another vulnerability…There are concerns that Chinese cable repair companies such as SBSS could tap undersea data streams.”
Erin Murphy, a Deputy Director at CSIS and expert on the undersea cable issue, likened the cable-repair issue to the questions any consumer might face when looking for a quick and effective fix.
“When you have a cable that needs to be repaired, you basically get in a queue to get a cable repair ship,” she told The Cipher Brief. “And sometimes it's Chinese. This doesn't mean that all Chinese ships are ready for espionage and ready for damage, but when there is a need to repair cables, you’ve got to go with the first-come, first-serve.”
Rear Adm. Studeman made the distinction between “outside-in” sabotage – the cable-cutting incidents – and “inside-out operations” that might be carried out in maintenance or repair work.
“The inside-out threat is just as significant and we need to be mindful of it,” Studeman said. Access to the cables, he said, allows U.S. adversaries to either capture data or sabotage the cables themselves.
“Part of it is about espionage and the ability to shunt information into a place where Chinese and Russian intelligence can go through it,” he said. “Even if it's encrypted, they're hoping that later on with decryption capabilities they are working on that they could end up having all this data that they can decrypt, and learn all sorts of secrets.”
Everyone needs a good nightcap. Ours happens to come in the form of a M-F newsletter that keeps you up to speed on national security. Sign up today.
What to do about the problem?
The recent congressional requests of the four tech juggernauts are essentially a probe of their exposure to undersea espionage. The letters went to those four companies for a good reason: Amazon, Google, Meta, and Microsoft own or lease roughly half of all undersea bandwidth.
The committees asked the companies to submit detailed information on the subsea cable systems they use – the companies that manufacture and maintain them, and whether any China-linked “system elements” are used in the cables. The tech firms were also asked to provide lists of entities that had been contracted to work on the cables since Jan 1, 2018, information about how they monitor the traffic of foreign-flagged vessels near the cables, and “the physical and cyber safeguards put in place by each company to protect the cables during this maintenance or repair.”
The letters referenced Russia as well, but the focus was on China. The committees requested answers by August 4, and a briefing from each company by August 8, 2025.
After the deadlines passed, a source close to the committees would say only that the tech firms had responded and that “we have meetings set up” on the issue. The Cipher Brief reached out to Meta, Microsoft, Google and Amazon for comment but we have not heard back from them.
Whatever the companies report, experts are convinced of the risks, and many have offered potential solutions. One obvious remedy would involve turning to American companies to do the maintenance and repair work. Experts have called for growth in the Cable Security Fleet program, through which Congress has funded two privately-owned U.S. ships to repair hundreds of cables that reach the U.S.
Among other ideas: Build more cables. As CSIS’s Murphy said, “It comes down to a redundancy issue. The more cables that you lay…the more redundancy you build in.” Others have suggested establishing “a cable corridor,” in which critical cables are concentrated, meaning commercial vessels know to avoid the area, and monitoring is relatively easy. The drawback is that a malign actor would presumably learn about the location of “corridor” as well.
Rear Adm. Studeman and others have suggested the use of technology to upgrade the cable network, ensuring that more undersea cables are “smart,” and equipped with sonar to detect breaks easily. He suggested that sensors be placed in cables that would detect anomalies and “indicate that somebody's up to no good.” Such anomalies might include signal distortions, latency delays, and any hints that repair work had been done in a questionable manner.
“All those things deserve to have more sensors and therefore more analysis and more awareness,” Studeman said, “because then you will know how to act appropriately to nip something in the bud, or to stop it soon after you detect it.”
Ultimately, the concerns about infiltrating undersea cables amount to one more worry for national security officials who are already concerned that China has breached a range of critical systems in the U.S.
“We know that China's inside our critical infrastructure in the United States,” Sanner said. “And they are there to pre-position themselves in case of war, or maybe even as a preemptive thing to prevent us from interfering in, for example, a Taiwan invasion. So I would think that undersea cables are no different when it comes to the United States.”
Are you Subscribed to The Cipher Brief’s Digital Channel on YouTube? There is no better place to get clear perspectives from deeply experienced national security experts.
Read more expert-driven national security insights, perspective and analysis in The Cipher Brief because National Security is Everyone’s Business.
