During a routine training drill, an Indonesian navy submarine crew of 53 men may have been struck by an unknown yet strong force that pulled them to the depths of the Bali Sea.

Indonesian navy officials believe the sinking of KRI Nanggala 402 and the deaths of its 53 crew is caused by an internal solitary wave, which is believed to occur in the seas around Bali.

Lombok Strait
(Photo: Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC)
On November 1, 2016, NASA’s Aqua satellite passed over Indonesia, allowing the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard to capture a stunning true-color image of oceanic nonlinear internal solitary waves from the Lombok Strait.

Radio New Zealand (RNZ) said the ship then sunk to a depth of 838 meters. It's well beyond rescuers' reach.

Another RNZ report added that there was no way everyone would survive when the crew members' personal belongings floated up and the oxygen supplies onboard steadily depleted.

What went wrong stayed the mystery.

Many hypotheses have been proposed. But officials now believe there is proof that an underwater wave - capable of exerting a strong vertical tug under the sea surface - existed in the Bali Sea about the time the submarine vanished last Wednesday morning.

 Internal Solitary Wave Explained

The Lombok Strait, which connects the islands of Bali and Lombok, is reputed to produce strong internal waves on a nearly fortnightly basis.

According to NASA, strong tidal waves, a harsh ocean floor, and the exchange of water between two channels - one shallow and one deep - tend to converge around every 14 days to produce a powerful tidal flow.

 Indonesian navy authorities said per ABC News that this natural occurrence is a more plausible cause for the submarine accident than other hypotheses proposed in recent days.

Navy authorities said per Straits Times photographs from Japan's Himawari 8 satellite and the European satellite Sentinel revealed massive underwater waves coincided with the sinking of the KRI Nanggala 402.

According to Rear Admiral Iwan Isnurwanto, Commander of the Indonesian Navy Command and Staff School, the waves came up from the bottom to the north, and there's a trench between two mountains.

The wave had a speed of two nautical miles and a volume of two to four million cubic meters of water.

According to Admiral Iwan, an internal wave could make the crew powerless in the face of nature. It will be nature's will, he said.

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Will Indonesia Be Able to Retrieve the Ship From the Bottom of the Sea?

Soon after being granted authorization to dive for a torpedo shooting drill, the KRI Nanggala 402 lost contact with navy officials around 4:00 a.m. on April 21.

A few hours later, search ships and a helicopter confirmed an oil spill and the scent of diesel fuel in the area.

Over the course of several days, ships from all over the world entered a vast search, and on Saturday, tell-tale items from the lost submarine, including prayer mats, part of the torpedo firing system, and oil for the sub's periscope, were discovered floating in the Bali Bay.

According to an underwater scan, the submarine had sunk to a depth of 838 meters and broken into at least three sections, according to an underwater scan: the hull, the main section, and the stern.

A remotely controlled camera captured the sub's final resting spot on the seafloor.

Given the logistics of such a mission and the enormous cost, Indonesian military officials are now debating how - or whether - they will lift the submarine from such a depth.

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