Lombok Strait

The Strait of Malacca is the front door between the Indian and Pacific Oceans — crowded, shallow, and famous. The Lombok Strait is the back door, and for certain ships it is the only door that fits. Running north–south between the Indonesian islands of Bali and Lombok, it is deep where Malacca is shallow, which makes it the passage of choice for the largest deep-draft tankers and bulk carriers — and, crucially, for submarines that wish to cross between oceans without ever coming up for air. In an era when China’s strategists lie awake over the vulnerability of their oil imports, the Lombok Strait is the most viable alternative to the chokepoint they fear most.

Geographic Position¶

Lombok runs roughly sixty kilometres between Bali to the west and Lombok island to the east, in the chain of the Lesser Sunda Islands, connecting the Indian Ocean in the south to the Java and Flores Seas — and thence the Pacific approaches — in the north. It narrows to around eighteen kilometres at its southern end, near the small islands off Bali, and widens to perhaps forty at the north.

Its defining feature is depth. Where Malacca’s controlling depth is around twenty-five metres, Lombok’s navigable channel runs to several hundred metres and exceeds a thousand in places — an order of magnitude deeper. That depth, and the strong currents that pour through it carrying a fifth of the Indonesian Throughflow (the great ocean current that drains the Pacific into the Indian Ocean), make Lombok both a highway for big ships and a complex, sometimes treacherous body of water. The same stratified, fast-moving waters generate powerful internal waves beneath the surface — a hazard, as it turned out, to submarines.

The Malacca Alternative¶

Lombok carries only a fraction of Malacca’s traffic in normal times — Malacca handles the overwhelming majority of the region’s shipping and, by the EIA’s recent reckoning, well over 20 million barrels of oil a day. But Lombok’s importance is not measured by ordinary throughput; it is measured by what it can do that Malacca cannot, and by what would happen if Malacca closed.

Two things distinguish it. First, depth: the very largest tankers and bulk carriers — vessels for which Malacca’s draft is marginal — are routinely routed through Lombok and onward via the Makassar Strait, especially iron-ore and coal carriers and traffic to and from Australia. Second, redundancy: if the Strait of Malacca were ever blocked, by accident, blockade, or war, the EIA estimates that a large share of the world’s shipping fleet would have to reroute around the Indonesian archipelago, and the deep, relatively unobstructed Lombok channel is the premier candidate to absorb that traffic. The detour adds days of steaming and considerable cost, but Lombok has the depth headroom that the shallower Sunda Strait lacks.

The Malacca Dilemma and the Submarine Question¶

This is where Lombok becomes strategy rather than geography. China imports the majority of its oil and conducts most of its seaborne trade through the Strait of Malacca — a dependence that Hu Jintao reportedly named the “Malacca dilemma” two decades ago: the fear that a hostile power could throttle China’s economy by closing a strait it does not control. Lombok is the most credible answer to that dilemma. It is China’s deep-water bypass — but it runs through Indonesian sovereignty and within easy reach of American, Australian, and Indian surveillance, so it trades one vulnerability for another.

The strait is also the spine of Indonesia’s second archipelagic sea lane, the legally recognised corridor — Makassar Strait to Flores Sea to Lombok — through which ships of all nations enjoy the right of passage. And here Lombok’s depth has a military edge that Sunda’s shallowness denies: it is one of the few Indonesian passages deep enough for a submarine to transit fully submerged, at operational depth, without surfacing. That makes it a key node in the undersea movement of the island-chain contest — for Chinese, American, and, in the AUKUS era, future Australian submarines crossing between the Indian Ocean and the South China Sea.

The competition over that undersea geography surfaced, almost literally, in 2026, when an Indonesian fisherman recovered a Chinese-made underwater glider in the northern Lombok Strait, fitted with sensors and a current profiler — the kind of oceanographic equipment that maps the very conditions a submarine needs to know to hide or to hunt. It was at least the third such device found in Indonesian waters, and analysts read it as exactly what it looked like: preparation of the underwater battlespace.

Krakatoa’s Quieter Cousin¶

Lombok lacks the catastrophic volcanic history of the Sunda Strait, but its waters have proven their dangers in other ways. In April 2021 the Indonesian Navy submarine KRI Nanggala-402 was lost with all fifty-three crew during an exercise near the northern approaches to the strait. The boat sank to around 838 metres — far below its crush depth — and broke apart, and the Navy’s leading explanation points to one of the powerful internal solitary waves that the strait’s stratified, current-driven waters generate: an invisible underwater swell capable of dragging a submarine down faster than it can recover. The cause remains a probable assessment rather than a proven verdict, but it is a reminder that the same depth that makes Lombok strategically valuable also makes it unforgiving.

Australia’s Lifeline¶

For one country in particular, the Indonesian straits are not an abstraction but an economic windpipe. The overwhelming majority of Australia’s maritime trade — by various estimates around 83 percent of its imports and close to 90 percent of its exports — passes through the archipelagic passages to its north, of which Lombok is among the most important. Iron ore and coal heading out, fuel and manufactured goods coming in: the prosperity of a G20 economy threads through a handful of Indonesian straits over which Canberra has no control and only limited visibility.

That dependence is the quiet logic behind much of Australia’s strategic anxiety, and behind AUKUS. The plan to field a fleet of nuclear-powered attack submarines is, at bottom, a bet on the ability to operate in and around exactly these waters — to patrol the deep approaches through which trade flows and through which a rival’s submarines would have to pass. Lombok’s depth, which lets a submarine transit submerged, makes it a natural focus of that effort: the passage where the undersea contest for the northern approaches would most plausibly be fought.

It also explains why the discovery of a Chinese underwater glider in the strait in 2026 set off alarms in Canberra as much as Jakarta. A device mapping the currents and acoustics of the Lombok channel is mapping the battlespace on Australia’s most important trade route — the environmental data that decides whether a submarine can hide there, and whether another can find it. For Australia, the Indonesian straits are the geography on which its security ultimately rests, and Lombok is the deepest and most contested door among them.

Strategic Assessment¶

Lombok is the chokepoint defined by its alternative. Its day-to-day traffic is modest, but its strategic weight comes from being the deep, viable bypass for the world’s most important shipping route — and the submerged corridor through the archipelagic barrier that separates the Indian Ocean from the Pacific. For China it is the hedge against the Malacca dilemma; for Australia, whose trade overwhelmingly transits the Indonesian straits, it is part of the northern maritime approaches that AUKUS submarines are meant to patrol; for Indonesia it is sovereign water through which it must, by law, allow passage while watching warily as great-power navies and their underwater drones take an ever-closer interest.

Conclusion¶

Most chokepoints earn their fame in peacetime, by the trade they carry. Lombok would earn its in a crisis, by the trade it could absorb and the submarines it can hide. It is the deep-water back door to the Pacific — the passage that matters most precisely when the front door is shut, and the stretch of Indonesian water where the quiet, patient contest for control of the undersea Indo-Pacific is already underway.

Sources & Further Reading¶

• “World Oil Transit Chokepoints” (US Energy Information Administration) — on Malacca’s dominance and the Indonesian straits as its principal alternatives.

• “What the Hormuz crisis reveals about Australia’s Indonesian sea lanes” by Aristyo Rizka Darmawan (Lowy Institute) — on the dependence of Australian and regional trade on Lombok and its neighbours.

• “Indonesian Navy Declares Missing Submarine Sunk” (USNI News) — the loss of KRI Nanggala-402 and the internal-wave hypothesis.

• “The Influence of Sea Power upon History” by Alfred Thayer Mahan — the foundational logic of why control of straits shapes the fate of fleets.