The Sunda Strait

What it is

The Sunda Strait separates the Indonesian islands of Sumatra to the northwest and Java to the southeast, linking the Indian Ocean directly to the Java Sea and, beyond it, the South China Sea. At its narrowest the strait measures approximately 24 kilometres across; its widest reach spans roughly 100 kilometres. The eastern approaches are shallow, with sections recording depths of only 20 metres, limiting access for deep-draft vessels. The active volcano Anak Krakatau sits in the northern half of the strait alongside a scattering of smaller islands; tidal currents are strong throughout.

In the global chokepoint hierarchy, the Sunda Strait is a secondary alternative route. The U.S. Energy Information Administration classifies it, alongside the deeper Lombok Strait, as a bypass option if the Strait of Malacca were disrupted. In practice, the Malacca route handles roughly 100,000 commercial transits annually and carries the dominant share of regional oil and container traffic. Vessels exceeding approximately 100,000 deadweight tonnes, including most VLCCs, avoid the Sunda Strait's shallow eastern approach and route through Lombok instead. Smaller tankers and general cargo ships that do transit Sunda add roughly 200 nautical miles to a Middle East-to-Northeast Asia voyage compared with the Malacca route, a cost penalty that limits its routine use to vessels physically suited to it.

History

The strait's modern military significance was established on the night of 28 February to 1 March 1942, when the Australian cruiser HMAS Perth and the US cruiser USS Houston engaged a Japanese amphibious task group attempting to land on Java. Both Allied cruisers were sunk; Japanese control of the strait then exposed northern Australia to air attack and contributed to the collapse of Dutch colonial authority in the East Indies within weeks.

Krakatau's position in the strait produced two distinct catastrophic events. The 1883 eruption killed over 36,000 people along surrounding coasts and generated a pressure wave detected globally. Anak Krakatau, the volcanic island that grew from the 1883 caldera, underwent a southwestern-flank collapse on 22 December 2018, triggering a tsunami that reached wave heights of 6.6 metres in Banten province. Indonesia's disaster management agency BNPB recorded 437 deaths and 31,942 injuries. Because the event was volcanically generated rather than seismically triggered, no warning was issued by existing systems.

Current state

Indonesia holds sole sovereign authority over the Sunda Strait, unlike the Strait of Malacca, which is jointly governed by Indonesia, Malaysia, and Singapore. Indonesian law treats the passage as part of its archipelagic waters; international law under UNCLOS Part IV recognises a right of archipelagic sea lane passage for all states. There is no Traffic Separation Scheme in force comparable to Malacca's; vessel coordination relies on Indonesian Navy (TNI AL) patrols and port-authority traffic management. The strait carries coal and palm-oil export volumes from Java and Sumatra's southern ports. As of early 2026, piracy and maritime robbery remain recorded risks, though incident rates are lower than in the wider Malacca Strait.

Relationships

Indonesia is the sole governing state and bears full responsibility for navigation safety and security. Anak Krakatau is under continuous monitoring by Indonesia's Centre for Volcanology and Geological Hazard Mitigation (PVMBG); its eruptive status directly affects transit safety and coastal populations on both sides of the strait. China, Japan, and South Korea are the primary destination economies for cargo rerouted through Sunda when Malacca is congested. Australia tracks the strait as part of its northern maritime approach, a concern formalised in Indo-Pacific defence planning.

What to watch

A Sunda Strait bridge connecting Sumatra and Java has been discussed in Indonesian development plans since the 1990s and has been periodically revived; if built, it would reshape overland freight patterns but would not affect maritime transit. Anak Krakatau's eruptive cycle, last elevated in 2022, remains the principal environmental risk to passage reliability and to coastal communities in Banten and Lampung provinces. Any sustained disruption to the Strait of Malacca, whether from conflict, piracy surge, or infrastructure failure, would immediately raise the Sunda Strait's strategic weight and transit volume, testing Indonesia's capacity to manage increased traffic under its archipelagic waters regime.