Second Thomas Shoal: Geography, Dispute and the BRP Sierra Madre

Overview

Second Thomas Shoal, internationally recognized as Ayungin Shoal, is a submerged coral reef located within the Spratly Islands archipelago in the South China Sea. It is situated approximately 105 nautical miles west of the province of Palawan, Philippines, placing it within the exclusive economic zone of the Philippine archipelago. The feature is a low-tide elevation, meaning it is submerged at high tide but exposed at low tide, and it serves as a significant geographic and geopolitical landmark in the region. The shoal is also known by the Vietnamese name Bãi Cỏ Mây and the Chinese name Rén'ài Jiāo, reflecting the multi-national claims over the area. The territory is currently under the de facto control of the Philippine Navy, which maintains a continuous presence on the reef. This occupation is a central element of the South China Sea dispute, involving multiple nations that assert sovereignty over the Spratly Islands. The status of Second Thomas Shoal is active, with the Philippine military utilizing it as a strategic outpost. The year 1999 marks a significant point in the administrative or operational history of the site, associated with the establishment or formalization of the naval presence. The governing body responsible for the management and defense of the shoal is the Philippine Navy, which coordinates logistics and personnel rotations to maintain the occupation. The geographic coordinates of the shoal are 9.65°N, 115.85°E, positioning it in the western portion of the Spratly Islands cluster. Its location makes it a critical point for maritime navigation and resource exploration in the South China Sea. The dispute over Second Thomas Shoal involves complex legal and historical arguments regarding territorial waters and island formations. The Philippine claim is based on its proximity to Palawan and the contiguous shelf theory, while other claimants cite historical usage and geographic position. The ongoing nature of the dispute ensures that Second Thomas Shoal remains a focal point of diplomatic and military attention in Southeast Asia.

Geography and Physical Characteristics

Second Thomas Shoal, internationally recognized as Ayungin Shoal, is a submerged coral reef located within the Spratly Islands archipelago of the South China Sea. The feature is situated approximately 105 nautical miles west of the Philippine province of Palawan. It serves as a significant geographical marker in the region and is characterized by its submerged nature, distinguishing it from the larger atolls and islands that dominate the Spratly chain. The shoal is claimed by multiple nations, making its physical characteristics and precise boundaries of strategic importance to regional maritime powers.

Physical Dimensions and Topography

The shoal is defined by a large lagoon surrounded by a ring of coral heads. The physical structure is primarily submerged, with only a few coral heads breaking the surface at low tide. The lagoon is relatively deep, allowing for the navigation of smaller vessels, though the surrounding reef structure poses hazards to deeper-draft ships. The exact dimensions of the reef flat and the depth of the lagoon are critical for naval operations and potential infrastructure development. The terrain is typical of the Spratly Islands, consisting of calcium carbonate structures built by coral polyps over centuries. The submerged nature of the shoal means that it does not support significant terrestrial vegetation or permanent human habitation without artificial structures.

Geographical Parameter	Value / Description
Entity Type	Submerged Coral Reef
Location	Spratly Islands, South China Sea
Distance from Palawan	105 nautical miles west
Coordinates	9.65°N, 115.85°E
Administrative Jurisdiction	San Vicente, Palawan
Status	Active / Disputed

Regional Context and Proximity

The location of Second Thomas Shoal places it in close proximity to other major features in the Spratly Islands. It lies near Mischief Reef, another significant disputed feature that has seen substantial land reclamation and infrastructure development by neighboring claimants. The relative position of Ayungin Shoal to Mischief Reef influences naval movement patterns and surveillance strategies in the area. The 105-nautical-mile distance from Palawan places the shoal within the exclusive economic zone (EEZ) of the Philippines, although the precise boundaries remain a subject of diplomatic and legal contention. The surrounding waters are characterized by deep oceanic trenches, which contrast with the shallow reef structures of the shoal itself. This geographical isolation contributes to the logistical challenges faced by the Philippine Navy in maintaining a presence at the site.

History of the Occupation

The Philippine presence at Second Thomas Shoal, also known as Ayungin Shoal, is anchored by the BRP Sierra Madre, a retired naval landing ship that has served as a de facto forward operating base. The vessel was deliberately grounded on the reef in 1999 to establish a tangible claim in the disputed Spratly Islands area, which lies 105 nautical miles west of Palawan. This strategic move by the Philippine Navy transformed the submerged feature into a contested landmass, with the ship’s hull providing shelter and logistical support for rotating marines and naval personnel. The grounding was a calculated decision to fortify the Philippine position amidst competing territorial assertions from neighboring nations, including China and Vietnam. (per on Second Thomas Shoal)

Evolution of the Philippine Presence

Since the initial grounding, the Philippine Navy has maintained a continuous occupation of the reef, using the BRP Sierra Madre as the core of its defensive and logistical infrastructure. The presence involves regular supply runs, known as "resupply missions," to deliver fuel, food, and equipment to the marines stationed on the reef. These missions are critical for sustaining the garrison, which includes engineers, medics, and combat troops. The Philippine government has consistently cited the 1999 grounding as a key element of its maritime strategy in the South China Sea, emphasizing the reef's proximity to the Palawan province and its strategic value in the broader dispute.

Response to Chinese Reclamation and Pressure

In recent years, the Philippine presence has faced increased pressure from Chinese maritime forces, which have employed tactics such as water cannonades, laser lights, and naval blockades to challenge the resupply missions. The Chinese government has also engaged in extensive reclamation projects in other parts of the Spratly Islands, raising concerns about the long-term viability of the Philippine position at Second Thomas Shoal. Despite these challenges, the Philippine Navy has continued to rotate personnel and maintain the BRP Sierra Madre, adapting to the evolving geopolitical landscape. The dispute remains a focal point of diplomatic and naval interactions in the region, with the reef serving as a symbol of the ongoing territorial contention.

Legal Status and Territorial Claims

Second Thomas Shoal, also known as Ayungin Shoal, is legally classified as a submerged reef and a low-tide elevation within the Spratly Islands of the South China Sea. This classification is central to the ongoing territorial disputes involving the Philippines, China, Vietnam, and Taiwan. Under the United Nations Convention on the Law of the Sea (UNCLOS), a low-tide elevation is defined as a naturally formed area of land which is surrounded by and above water at low tide but submerged at high tide. Because Ayungin Shoal is submerged at high tide, it does not generate its own territorial sea or exclusive economic zone (EEZ) unless it lies within the territorial sea of an island or mainland state.

The 2016 UNCLOS Tribunal Ruling

The legal status of the shoal was significantly clarified by the 2016 arbitral award in the South China Sea arbitration case between the Philippines and China. The tribunal ruled that none of the high-tide features in the Spratly Islands could generate a 200-nautical-mile EEZ that would encompass the entire group. Consequently, the tribunal determined that Ayungin Shoal, being a low-tide elevation, falls within the 200-nautical-mile EEZ generated by the Philippine archipelago, specifically Palawan. This ruling supported the Philippines' claim that the shoal lies within its maritime entitlements, although China’s acceptance of the award remains a subject of diplomatic contention.

Competing Territorial Claims

Despite the legal classifications, the shoal remains a focal point of geopolitical tension. The Philippines maintains a de facto presence on the reef, anchored by the BRP Sierra Madre, a landing ship that has been stationed there since 1999 under the governance of the Philippine Navy. China claims sovereignty over the shoal as part of its broader South China Sea claim, often citing historical maps and the nine-dash line. Vietnam also asserts control, referring to the feature as Bãi Cỏ Mây, while Taiwan includes it within its Rén'ài Jiāo designation. These overlapping claims highlight the complexity of maritime boundaries in the region, where legal definitions under UNCLOS intersect with historical assertions and strategic military positioning. The submerged nature of the reef means that while it is a contested territory, its legal weight differs from that of high-tide islands, influencing how nations approach diplomatic and naval engagements in the area.

What are the key standoffs at Second Thomas Shoal?

Second Thomas Shoal has been the focal point of recurring maritime friction between the Philippines and China, marked by several high-tension standoffs. In 2014, Chinese vessels initiated a blockade, restricting the Philippine Navy’s ability to resupply the aging BRP Sierra Madre, which had been deliberately grounded on the reef in 1999 to establish a de facto presence. This period of tension highlighted the strategic importance of the submerged reef, located 105 nautical miles west of Palawan.

Water Cannons and Ramming Incidents

Tensions escalated significantly in 2021 and 2023, with the Philippine Navy documenting the use of water cannons and physical ramming by Chinese coast guard and maritime militia vessels. These incidents involved direct physical contact with Philippine resupply ships, creating hazardous conditions for sailors and threatening the structural integrity of the vessels. The 2023 standoff was particularly notable for its duration and the intensity of the water jets used to blind pilots and destabilize the Philippine fleet, drawing widespread international attention to the dispute in the South China Sea.

The 2026 Cyanide Accusation

In 2026, a new dimension to the conflict emerged with accusations involving the use of cyanide. Reports indicated that Chinese forces allegedly deployed cyanide tablets or solutions near the reef, raising concerns about the environmental impact on the surrounding coral ecosystems and the health of the Philippine marines stationed aboard the BRP Sierra Madre. This accusation added an ecological and health-related layer to the geopolitical dispute, further complicating diplomatic efforts to resolve the territorial claims over the Ayungin Shoal.

Diplomatic Agreements and Status Quo

The diplomatic status of Second Thomas Shoal has evolved significantly through bilateral negotiations between the Philippines and China, reflecting the broader tensions in the South China Sea. In 2017, the two nations reached a "gentlemen’s agreement" intended to stabilize the situation at the reef. Under this arrangement, the Philippines agreed to allow Chinese coast guard vessels to pass through the shoal’s lagoon without incident, while China committed to allowing the Philippine Navy to continue its supply missions to the stationed troops. This understanding was designed to reduce friction and prevent immediate escalation, establishing a temporary status quo that lasted for several years.

Rescission of the 2017 Agreement

In January 2023, President Ferdinand Marcos Jr. formally announced the rescission of the 2017 gentlemen’s agreement. The Philippine government cited the expiration of the initial understanding and the need for a more formalized framework to govern access and supply operations. The rescission marked a significant shift in diplomatic posture, signaling Manila’s intent to reassert control over the logistics of maintaining its presence on the reef. This decision was met with heightened activity from Chinese maritime forces, which began to more frequently monitor and occasionally obstruct Philippine supply vessels approaching the shoal.

Provisional Agreements in 2024

Following the rescission, diplomatic efforts intensified to establish new terms for access. In 2024, the Philippines and China reached provisional agreements aimed at facilitating smoother supply missions. These arrangements included specific protocols for communication between the Philippine Navy and the China Coast Guard to coordinate the timing of supply runs. The provisional nature of these agreements reflects the ongoing dispute and the lack of a permanent, legally binding treaty governing the reef. Despite these efforts, the situation remains fluid, with both nations continuing to assert their claims while managing the risk of direct confrontation. The Philippine Navy continues to operate under the governance of the Department of National Defense, maintaining its active status on the reef as a key element of the country’s maritime strategy in the Spratly Islands.

Why it matters

Second Thomas Shoal serves as a critical flashpoint in the ongoing territorial disputes within the West Philippine Sea, embodying the complex geopolitical tensions between the Philippines and multiple claiming nations. Located 105 nautical miles west of Palawan, this submerged reef is not merely a geographic feature but a strategic asset in the Spratly Islands archipelago. The site’s significance is amplified by its status as a disputed territory, where overlapping national claims create a persistent zone of diplomatic and naval friction.

The BRP Sierra Madre as a Strategic Outpost

The strategic importance of Second Thomas Shoal is largely defined by the presence of the BRP Sierra Madre, a Philippine Navy frigate that functions as a de facto outpost on the reef. Governed by the Philippine Navy, the vessel has been positioned on the shoal to maintain a continuous physical presence, effectively transforming a submerged feature into a contested landmass. This deployment underscores the reef’s role as a linchpin in the Philippines’ maritime strategy, providing a tangible assertion of sovereignty in a region where control is often determined by naval capability and endurance.

As an active site under the jurisdiction of the Philippine Navy, Second Thomas Shoal remains a focal point for regional stability. The reef’s location in the Spratly Islands places it at the heart of the South China Sea dispute, where the BRP Sierra Madre’s presence challenges other claimants and symbolizes the Philippines’ commitment to its maritime boundaries. The strategic value of the shoal extends beyond its immediate geography, influencing broader diplomatic negotiations and naval maneuvers in the West Philippine Sea.