JAKARTA -- Indonesia's navy on Tuesday said that last week's fatal submarine sinking was likely caused by an underwater phenomenon known as an "internal solitary wave."
Officers said that differences in the density of waters off Bali and in the nearby Lombok Strait may have triggered a "massive movement" strong enough to pull down the submarine in seconds.
Iwan Isnurwanto, commander of the Navy Staff and Command School, said the presence of the wave around the location of the submarine at the time of the accident last Wednesday was confirmed from images produced by Japanese weather satellite Himawari 8.
"There was nothing that they could do, no time to do anything... if the sub was brought down by such a wave. It likely angled [downward], causing all the crew members to roll down [to the bottom of the vessel]," Isnurwanto told a news briefing at navy headquarters in Jakarta. "We have to do further investigation, but that is most likely what happened."
With large amplitudes that cause powerful currents, internal solitary waves are considered a major hazard to marine engineering and submarine navigation. They can impose unexpectedly large stresses on offshore oil rigs.
Naval officials said more surveys to detect potential internal solitary waves in Indonesia's waters will be needed to avoid similar incidents in future submarine operations.
Officials dismissed speculation about other possible causes. These included allegations of poor maintenance of the aging submarine, human errors, as well as a rumor circulating on social media that it had been shot by a foreign vessel.
The KRI Nanggala 402 is a German-built submarine that had been in service since 1981, and underwent a full refit in South Korea that was completed in 2012.
Muhammad Ali, assistant to the navy chief of staff, said the submarine had undergone regular checks, including its last "docking" last year, when the vessel was deemed seaworthy through September 2022.
He added all crew members on board the vessel were well trained, and it was not true that the submarine was over its crew capacity. The vessel was earlier reported to have a capacity of just 33 people, but Ali said that represented the number of beds available, and that subs operated by the navy commonly carried more than 50 personnel.
"[The rumor] that it had been shot by a passing foreign vessel is I think outrageous," Ali added. "We had many above-water ships during the time of the incident, and they have sonars, which could have detected an explosion if it happened."
The Nanggala vanished in the early hours of Wednesday while conducting a torpedo-firing exercise. The last detected signal from the vessel was from a depth of 850 meters, beyond its diving limit.
The last communication with the sub was at 4 a.m. on Wednesday. When the commander of the training task force tried to authorize the firing drill 25 minutes later, communication with the submarine could not be established. The vessel had been due to surface by 5:15 a.m. on Wednesday morning.
On Sunday, the Indonesian navy, with support from Singapore's MV Swift Rescue, a submarine support and rescue vessel, found large pieces of debris at a depth of 838 meters. The vessel was believed to have split into three major parts, including the ruptured main body, eroding all hopes that survivors would be found.
The navy said attempts to evacuate the sub and remains of the victims will continue, though it has not been decided how they were going to salvage the wreckage. The MV Swift could only lift smaller parts.
Navy deputy chief of staff Ahmadi Heri Purwono said Indonesia now has just four submarines to safeguard the vast archipelago. One, similar to the Nanggala, is also German built and has been in service for 40 years, and three newer ones were made in South Korea.
The navy said it was planning to procure a submarine rescue vessel following the incident.
But asked whether the purchase of more submarines will be accelerated, Purwono only said, "Let's pray we can have more submarines in the future."