Amid the recent political turmoil in South Korea following President Yoon Suk Yeol’s December 2024 martial law declaration and subsequent impeachment, tensions between the Republic of Korea (ROK) and People's Republic of China (PRC) have also escalated in early 2025 over Beijing's installation of large "steel structures" in the Yellow Sea's Provisional Measures Zone (PMZ)--a huge maritime area where the two countries' exclusive economic zones (EEZs) overlap. The dispute featured diplomatic posturing, a tense maritime standoff and competing claims about the structures' true purpose--but what exactly are these structures? 

Thanks to some extensive sleuthing and help from several open-source intelligence partners, SeaLight was recently able to crack the code, leading to this just-published story in Newsweek.

Maritime Standoff

At 14:30 Korea Standard Time on February 26th, the ROK’s R/V Onnuri research vessel, escorted by Korea Coast Guard (KCG) 3010, approached the Shen Lan 2 Hao (or Deep Blue 2)—a 71.5 meter tall underwater cage deployed by China into the PMZ to conduct salmon aquaculture and harvesting. The ROK government sent the Onnuri out to inspect the structure, as it believes is its right under the 2001 Korea-China Fisheries Agreement, which constrains activities in the zone except as mutually agreed. 

Korea was concerned not only solely the cage, but also about a massive jack-up support rig China has deployed nearby. Very few people have seen this rig until now, but SeaLight--with help from its imagery partner SkyFi--has now identified and imaged this behemoth for the world to see:

The rig measures approximately 108 meters long (from helipad to its opposite tower) and 82 meters broad. Based on SeaLight's analysis of past Planet Labs imagery, this rig was first deployed in October 2022. The new rig replaced a smaller rig that was sent out at least two years before to join Shen Lan 2 Hao's own smaller predecessor, Shen Lan 1 Hao, which was first deployed in 2018 to much fanfare:

The Shen Lan 2 Hao, with three times the capacity of Shen Lan 1 Hao, has only recently attracted the attention of the ROK government, though its launch was announced in early 2024.

With the help of our friends at Planet Labs, SeaLight was able to acquire a clear image the deployment of the Shen Lan 2 Hao with its support rig and vessel on the day of the 26 February incident, just hours before the arrival of the Onnuri, its KCG escort and the CCG response force.

China’s coast guard ships reacted immediately to the Onnuri's mission. The Qingdao-based China Coast Guard (CCG) 6402, together with the patrolling CCG 6305 and three civilian-crewed rubber boats intercepted the Onnuri, blocking its path to the rig. Chinese civilians aboard the boats--some reportedly armed with knives--shouted demands to leave, while KCG 3010 positioned itself as a buffer during the 2-hour impasse. 

SeaLight was able to reconstruct the event with the help of our friends at Starboard Maritime Intelligence:

This interactive data visualization created by SeaLight Tech Advisory Group Director Anna van Amerongen shows how the incident unfolded (use the slider to pause, advance, reverse or adjust the view options):

Advance in Aquaculture or Gray-Zone Aggression?

The PRC has hailed Shen Lan 2 Hao as a breakthrough in sustainable aquaculture. Operated by the state-linked Shandong Marine Group, it features automated feeding systems, underwater imaging and remote-control capabilities, enabling salmon farming in 9,000 caged cubic meters of water. 

Under UNCLOS Article 60, coastal states have jurisdiction over artificial installations within their EEZs, but the PMZ’s overlapping claims and bilaterally negotiated regulations create ambiguity. Does this unilateral deployment of huge "steel structures" within contested waters signal a bid to strengthen China's claim and exert de facto control?

Lee Dong-gyu, a research fellow at the Asian Institute for Policy Studies, enunciated the ROK concerns: "China's increasing fishing activities and military exercises in South Korea's EEZ could be used to establish its effective control over the West Sea and gain leverage in negotiations with South Korea." Seoul lodged a formal protest and summoned a Chinese embassy official, asserting its right to investigate the structure under the 2001 Korea-China Fisheries Agreement.

This agreement established a joint fisheries management regime, overseen by a bilateral committee, specifically to avert such controversies. China, however, apparently ignored this process when it first deployed the Shen Lan 1 Hao with a support ship in 2018, as it has continuously through the deployment of both support rigs and now the Shen Lan 2 Hao.

Beijing counters that the rig operates within its own EEZ, ignoring the 2001 agreement altogether. Its embassy in Seoul asserted the installations complied with domestic and international law. China has also defended the structures as environmentally responsible, claiming they adhere to strict protocols to mitigate risks to navigation safety and marine ecosystems. Ambassador Dai Bing reinforced this narrative during diplomatic engagements, in which he has repeatedly framed the operations as benign aquaculture projects and the ROK's reaction as hyperbolic.

A more cynical view is that this is an early step in bringing a South China Sea-style gray zone campaign of "salami slicing" to the Yellow Sea. In this view, the incremental deployment of rigs of increasing size into the PMZ reflects a recognizable pattern of testing rival states’ resolve with ostensibly civilian and coast guard presence while avoiding overt militarization. At the same time, Beijing's policy of civil-military fusion leaves open the possibility that activities unrelated to salmon harvesting--such as intelligence collection, for example--might also be going on somewhere within China's giant new support rig. 

What's next?

Last month, Seoul responded by deploying its own stationary floating platform into the contested zone to conduct "environmental surveys" and monitor Chinese activities, signaling its intent to challenge Beijing’s unilateral claims through its own presence operations. Diplomatically, the Ministry of Foreign Affairs intensified bilateral engagement, asserting South Korea’s “legitimate maritime rights” under the 2001 agreement and international law and coordinating cross-agency efforts to address the structures’ legal ambiguities. Publicly, domestic frustration culminated in a rally by the Korean Veterans Association outside the Chinese Embassy in Seoul, where protesters denounced the installations as threats to regional security and demanded their immediate removal.

South Korea faces a policy dilemma as it navigates competing pressures on both domestic and international fronts. Domestically, lawmakers are demanding sanctions against Chinese firms linked to the unauthorized rigs. However, these calls clash with economic realities: China remains South Korea’s largest trading partner, making punitive measures politically and financially fraught. Simultaneously, Seoul’s efforts to strengthen military coordination with the U.S.—including joint drills in the Yellow Sea aimed at countering Chinese assertiveness—risk provoking Beijing and destabilizing bilateral relations at a time when recently announced U.S. tariffs constrain Seoul's options.

The Korea-China PMZ’s overlapping claims extend beyond surface-level fisheries disputes, encompassing strategic resource competition and maritime mobility risks. The zone overlaps with potentially significant undersea oil and gas reserves, including estimates of 1.1 billion barrels of oil and 2.2 trillion cubic feet of gas in the West Korea Bay–North Yellow Sea Basin, heightening stakes for both nations. Simultaneously, China’s increased coast guard patrols near the 124th meridian east—a boundary Seoul asserts as its EEZ limit—threaten to disrupt critical shipping lanes. 

Former International Tribunal for the Law of the Sea official Kim Doo-young has warned that China’s grid-like placement of large steel structures in the Yellow Sea could effectively block maritime access, complicating any legal resolution to the dispute. Professor Nam Sung-wook of Korea University has further speculated that these installations may be intended to restrict U.S. military operations in the event of a Taiwan Strait conflict, noting their alignment with strategic locations such as Pyeongtaek, home to U.S. Forces Korea. 

The current state of affairs highlights the escalating risk that maritime disputes in the Yellow Sea--like those in the South and East China Seas--could spiral into broader regional confrontations, especially as China’s assertiveness grows and the U.S. shifts some attention to the second island chain. The turn of events underscores the Yellow Sea’s transformation from a zone of shared economic activity into a potential flashpoint for great power rivalry, where the stakes for regional security and freedom of navigation are higher than ever.