Someone is showing a lot of underwater interest in Indonesia’s straits

Indonesian fishers recovered three curious unidentified underwater objects from the country’s waters in April.

First, on 6 April, fishers found a 3.7-metre, torpedo-shaped device north of Gili Trawangan in the Lombok Strait. It bore the logo of the China Shipbuilding Industry Corporation.

A little over a week later, other fishers recovered a blue, elongated object near the Tanakeke Islands off South Sulawesi, between the Lombok and Makassar straits. And on 20 April a yellow torpedo-shaped device roughly two metres long was found off Kangean Island, just north of the Lombok Strait.

The significance of these discoveries lies in their location and recurrence. All were found in or near Indonesia’s Archipelagic Sea Lane II, the corridor connecting the Indian and Pacific oceans through the Makassar and Lombok straits

There were earlier finds, too: a Chinese Sea Wing underwater glider was recovered near the Masalembu Islands in January 2020, a little west of the direct line between the straits, and another near Selayar Island to the east of the line in December of that year. So these, too, were close to the sea lane.

Indonesian government investigations into the latest finds remain ongoing. Indonesia may also find it difficult to respond decisively, partly because international rules governing uncrewed submerged transit through designated archipelagic sea lanes remain unsettled.

The broader implication is nonetheless hard to miss: these waters are strategically significant far beyond their commercial function. Indeed, the Lombok Strait’s depth of more than 250 metres makes it one of the few passages in the region suitable for submerged transit. The Makassar Strait extends that route northwards towards the Celebes Sea and the Western Pacific with sufficient depth for underwater transit. Together, these straits provide a viable deep-water corridor for submerged movement between the Indian and Pacific oceans.

The recurrence of such discoveries suggests sustained interest in collecting data in and around the sea lane. A better understanding of the shape of the seafloor, for example, can help a commanding officer decide how fast a submarine can safely move. Salinity variations can cause a submarine to suddenly rise or sink, so knowing where and when those variations occur also helps navigation. An understanding of currents is similarly helpful.

The same information can also help predict where an adversary submarine is likely to operate. Temperature changes affect sonar performance, as does the nature of the seafloor, which can also determine where and whether mines can be laid. Collecting such data requires sustained observation over time, a task well suited to long-endurance underwater drones or static seabed sensors.

Several features of the straits make them especially attractive for such underwater data collection. First, they contain oceanographic hazards, including internal solitary waves, which can cause submarines to change depth abruptly. Second, they are strategic bottlenecks through which submarines may have to pass. Third, their limited size makes detailed data collection more practicable than in the open ocean or even in the South China Sea. The Lombok Strait, for example, is only around 40 km wide at its broadest point.

The uncrewed underwater objects themselves are difficult to detect, being so small and, powered by batteries, making little or no noise. And importantly, they are proliferating, fast.

Indonesia should expect more of them in its narrow and strategically important waterways.