Sovereignty, Passage, and the Archipelagic Regime under UNCLOS

For centuries, the classical law of the sea was dominated by a binary division: land territory, where states enjoyed absolute sovereignty, and the high seas, which were free for use by all nations. This traditional dichotomy struggled to address the unique geographic and geopolitical realities of mid-ocean island groups. For nations composed entirely of vast, sprawling islands dispersed across expansive oceanic regions, the sea was not a barrier separating distinct landmasses, but the very fabric holding their national territory together.

During the mid-20th century, emerging island nations, particularly Indonesia and the Philippines, argued that their territorial integrity and security required a legal regime recognizing their archipelagos as unified political and economic entities. This advocacy led to one of the most innovative and hard-fought chapters of the Third United Nations Conference on the Law of the Sea (UNCLOS III, 1973–1982). The result was Part IV of the United Nations Convention on the Law of the Sea (UNCLOS), which established the legal status of the “Archipelagic State” and created a complex regulatory framework balancing coastal state sovereignty with the freedom of international navigation. At the heart of this framework lies the regime of Archipelagic Sea Lanes Passage (ASLP).

The Legal Criteria: Defining the Archipelagic State

Under UNCLOS Article 46, an “archipelagic State” is defined as a State constituted wholly by one or more archipelagos and may include other islands. An “archipelago” is further defined as a group of islands, including parts of islands, interconnecting waters, and other natural features which are so closely interrelated that such islands, waters, and other natural features form an intrinsic geographical, economic, and political entity, or which historically have been regarded as such.

This definition deliberately excludes continental states that possess offshore archipelagos. For example, nations such as Greece, Canada, or Ecuador cannot claim archipelagic state status for their offshore island chains (such as the Aegean Islands, the Arctic Archipelago, or the Galápagos Islands, respectively). Only states composed entirely of islands, such as Fiji, the Bahamas, Indonesia, Papua New Guinea, and the Philippines, can invoke the provisions of Part IV.

The Mathematical Rigor of Article 47 Baselines

To qualify for and establish archipelagic status, a state must draw straight archipelagic baselines joining the outermost points of the outermost islands and drying reefs of the archipelago. However, UNCLOS does not grant unlimited discretion in drawing these lines. To prevent excessive maritime claims, Article 47 imposes strict mathematical and geographic constraints:

• The Water-to-Land Ratio: The ratio of the area of the water to the area of the land, including atolls, must be between 1:1 and 9:1. This ensures that the state is genuinely archipelagic. If the water area is too small relative to the land (under 1:1), the territory behaves more like a continental landmass. If the water area is too large (over 9:1, such as in the case of tiny, isolated islands separated by thousands of miles of open ocean), it cannot reasonably be enclosed as a single unit.
• Baseline Length Limits: The length of such baselines must not exceed 100 nautical miles, except that up to 3 percent of the total number of baselines enclosing any archipelago may exceed that limit, up to a maximum length of 125 nautical miles.
• General Configuration: The drawing of such baselines must not depart to any appreciable extent from the general configuration of the archipelago.

The establishment of these baselines is a highly technical hydrographic process. Once established, they serve as the inner boundary from which the state’s territorial sea, contiguous zone, exclusive economic zone (EEZ), and continental shelf are measured outward.

The Legal Status of Archipelagic Waters vs. Internal Waters

The waters enclosed within these archipelagic baselines are designated as “archipelagic waters.” Article 49 of UNCLOS explicitly establishes that the sovereignty of an archipelagic state extends to these waters, regardless of their depth or distance from the coast. This sovereignty also extends to the airspace above the archipelagic waters, as well as to their bed, subsoil, and the resources contained within them.

It is vital to distinguish archipelagic waters from “internal waters.” Internal waters, such as lakes, rivers, ports, and certain historic bays, are subject to the absolute territorial sovereignty of the coastal state, with no general right of foreign passage. Within archipelagic waters, however, Article 50 permits the archipelagic state to draw closing lines for the delimitation of internal waters (such as across river mouths, bays, and ports) in accordance with the standard rules of UNCLOS.

The waters outside these specific internal closing lines, but inside the outer archipelagic baselines, remain archipelagic waters. In these waters, the absolute sovereignty of the coastal state is qualified by explicit international navigational rights. This compromise was necessary to secure the acceptance of the archipelagic regime by maritime powers, who feared that enclosing vast bodies of water would obstruct international shipping and naval mobility.

The Right of Innocent Passage in Archipelagic Waters

Subject to the establishment of archipelagic sea lanes, ships of all states enjoy the right of innocent passage through archipelagic waters under Article 52 of UNCLOS. Innocent passage is defined as passage that is continuous and expeditious, and not prejudicial to the peace, good order, or security of the coastal state.

However, the right of innocent passage in archipelagic waters is subject to two critical limitations:

1. Suspension: The archipelagic state may, without discrimination in form or in fact among foreign ships, suspend temporarily in specified areas of its archipelagic waters the innocent passage of foreign ships if such suspension is essential for the protection of its security. Such suspension takes effect only after having been duly published.
2. Submarines and Airspace: Foreign submarines and other underwater vehicles navigating under the regime of innocent passage are required to navigate on the surface and to show their flag. Furthermore, innocent passage applies exclusively to maritime vessels; there is no right of innocent overflight through the airspace above territorial seas or archipelagic waters under the innocent passage regime.

The Core of the Compromise: Archipelagic Sea Lanes Passage (ASLP)

During the negotiations of UNCLOS III, maritime and military powers, particularly the United States and the Soviet Union, made it clear that they would not sign any treaty that allowed island nations to unilaterally close or restrict transit through critical international straits and choke points. Key sea lanes passing through the Indonesian and Philippine archipelagos were vital for global trade and naval deployment. In response, a unique legal mechanism was designed: Archipelagic Sea Lanes Passage (ASLP), codified in Article 53.

ASLP is a much more robust right of navigation than innocent passage. It is defined as the exercise of the rights of navigation and overflight in the normal mode solely for the purpose of continuous, expeditious, and unobstructed transit between one part of the high seas or an exclusive economic zone and another part of the high seas or an exclusive economic zone.

Defining the “Normal Mode” of Transit

The inclusion of the term “normal mode” is one of the most legally significant aspects of Part IV. While not explicitly defined in the text of the treaty, state practice and negotiating history indicate that “normal mode” means transiting in the manner that is customary for that specific type of vessel or aircraft. This yields several critical implications:

• Submerged Transit: Submarines are permitted to transit archipelagic sea lanes submerged, as it is their normal mode of operation. Under innocent passage, they would be required to surface.
• Overflight: Military and civil aircraft enjoy the right of overflight through the airspace corridors above designated sea lanes. No such right exists under innocent passage.
• Formation Flying and Defensive Maneuvers: Navies may transit in tactical formations, launch and recover aircraft, and operate sensors, provided these activities are directly related to the safe, continuous, and expeditious transit of the fleet.
• No Suspension: Unlike innocent passage, ASLP cannot be suspended or hampered by the archipelagic state for any reason, including security exercises, provided the transiting vessels adhere to international safety and environmental regulations.

Comparative Framework: Innocent Passage, Transit Passage, and ASLP

To understand the nuances of ocean governance, it is useful to compare the three primary navigational regimes established under UNCLOS:

The Mechanics of Designation: The Role of the IMO

Article 53(1) gives archipelagic states the right to designate sea lanes and air routes suitable for the continuous and expeditious passage of foreign ships and aircraft through or over their archipelagic waters and adjacent territorial seas. They may also prescribe traffic separation schemes (TSS) for the safe passage of ships through narrow channels.

However, the designation process is not unilateral. Article 53(9) mandates that an archipelagic state must refer proposals for designating or substituting sea lanes, or for prescribing or substituting traffic separation schemes, to the competent international organization with a view to their adoption. The recognized competent international organization for this purpose is the International Maritime Organization (IMO).

The IMO can only adopt such sea lanes and traffic separation schemes as may be agreed with the archipelagic state. This creates a dual-key system: the archipelagic state cannot formally designate a lane without IMO approval, and the IMO cannot impose a route without the state’s consent. This ensures that both global shipping interests and the coastal state's environmental and national security concerns are addressed.

The Default Rule: The Axis of Normal Routes

A crucial safety valve exists in Article 53(12): if an archipelagic state does not designate sea lanes or air routes, the right of archipelagic sea lanes passage may be exercised through the routes normally used for international navigation.

These routes are defined by a series of continuous axis lines from the entry points of passage routes to the exit points. Transiting vessels and aircraft must not deviate more than 25 nautical miles to either side of such axis lines during passage, with the caveat that they must not navigate closer to the coasts than 10 percent of the distance between the nearest points on islands bordering the sea lane.

State Practice: Case Studies of Indonesia and the Philippines

The practical implementation of the archipelagic regime is best understood through the contrasting experiences and legal approaches of two of the world’s largest archipelagic states: Indonesia and the Philippines.

Indonesia’s Three Sea Lanes

Indonesia was a pioneer of the archipelagic concept, first declaring its sovereignty over its unifying waters in the Djuanda Declaration of 1957. Following the ratification of UNCLOS, Indonesia worked systematically to designate formal archipelagic sea lanes.

In 2002, the IMO adopted Indonesia's proposal for three north-south archipelagic sea lanes (ASLs), known as ASL I, II, and III:

• ASL I: Passes through the Sunda Strait, Karimata Strait, and Natuna Sea.
• ASL II: Passes through the Lombok Strait and Makassar Strait.
• ASL III: Serves the eastern part of the archipelago, with branches connecting the Molucca Sea, Halmahera Sea, Banda Sea, Savu Sea, and Timor Sea.

While this was a milestone, it also generated international debate. Indonesia designated only north-south routes, choosing not to propose an east-west route through the Java Sea. Maritime nations argue that because an east-west route is a route normally used for international navigation, the default rule of Article 53(12) applies, granting them the right to conduct ASLP through the Java Sea despite the lack of formal designation. Indonesia, meanwhile, prefers that east-west traffic utilize the designated north-south lanes or operate under the more restrictive regime of innocent passage.

The Philippines’ Legal Landscape

The Philippines presents a unique constitutional and legislative history regarding archipelagic waters. The 1987 Constitution of the Philippines maintains a definition of national territory that historic claims associate with the boundaries established by the 1898 Treaty of Paris. Under this interpretation, the waters within these boundaries were viewed as internal waters, where no automatic right of innocent passage existed for foreign vessels.

To align its domestic legislation with its international obligations under UNCLOS, the Philippines enacted Republic Act No. 9522 in 2009. This Act redefined the country’s baselines, clearly distinguishing between its internal waters and its archipelagic waters. In doing so, the Philippines brought its maritime zones into compliance with international law, while reserving the right to regulate passage within its waters to protect its fragile marine ecosystems and national security interests.

Contemporary Security, Environmental, and Climate Challenges

The archipelagic regime is not a static legal compromise; it continues to face pressures from evolving maritime technology, shifting geopolitical alliances, and environmental change.

Environmental Vulnerability and Particularly Sensitive Sea Areas (PSSAs)

Most archipelagic states are located in highly biodiverse tropical regions. Enclosed waters, such as the Coral Triangle in Southeast Asia, host some of the most sensitive coral reef ecosystems on Earth. The passage of large oil tankers, container ships, and vessels carrying hazardous chemicals poses constant risks of catastrophic spills, ship strikes on marine mammals, and acoustic pollution.

Under UNCLOS, archipelagic states have the right to enforce laws and regulations relating to the prevention, reduction, and control of pollution from ships (Article 54, referencing Article 42). However, these regulations cannot have the practical effect of denying, hampering, or impairing the right of ASLP. To resolve this, states are increasingly turning to the IMO to designate specific regions within their archipelagic waters as Particularly Sensitive Sea Areas (PSSAs), allowing them to implement strict routing measures, mandatory reporting systems, and pilotage requirements.

Naval Geopolitics and Maritime Domain Awareness

As geopolitical rivalries intensify in the Indo-Pacific region, archipelagic waters have become critical strategic corridors. The ability of naval forces to transit through these choke points undetected or unhindered is a key component of naval strategy. For archipelagic states, monitoring these vast expanses of water to ensure transiting vessels comply with the “continuous and expeditious” requirement of ASLP is a daunting task.

Many developing island nations lack the maritime domain awareness (MDA) assets—such as radar networks, satellite tracking, and offshore patrol vessels—necessary to monitor underwater transit or high-altitude military overflight. This creates security vulnerabilities and raises concerns about unannounced military surveys or intelligence-gathering activities inside archipelagic waters, which the coastal state may deem contrary to the peaceful character of transit.

The Threat of Sea-Level Rise

A more fundamental, existential threat to the archipelagic regime is climate change. Because archipelagic baselines are anchored to the outermost points of islands, low-tide elevations, and drying reefs, rising sea levels pose a direct threat to these basepoints. If a critical reef or low-lying islet is permanently submerged, the baseline anchored to it may no longer meet the technical requirements of Article 47.

This could theoretically alter the water-to-land ratio or invalidate baseline segments, shrinking the state's archipelagic waters and moving the outer limit of its territorial sea inward. To address this, there is a growing consensus among international legal scholars and regional bodies, such as the Pacific Islands Forum, that maritime zones and baselines, once established in accordance with UNCLOS and deposited with the UN Secretary-General, should remain fixed and legally permanent, notwithstanding physical changes caused by sea-level rise.

Conclusion: A Delicate Balance of Global Maritime Order

Part IV of UNCLOS represents one of the most successful diplomatic compromises in the history of international law. By recognizing the concept of the archipelagic state, the international community acknowledged the deep geographical, historical, and economic connection that island populations have with the seas that surround them. At the same time, by establishing the robust navigational regime of Archipelagic Sea Lanes Passage, the convention preserved the vital interests of the global community in the freedom of navigation and overflight.

As maritime traffic grows, naval technologies advance, and environmental crises intensify, the resilience of this legal framework will continue to be tested. The ongoing stability of the global maritime order depends on the mutual commitment of both archipelagic states and seafaring nations to respect the delicate balance codified in Part IV: safeguarding the sovereignty and security of island nations while keeping the arterial waterways of the global economy open, safe, and free.