The Indonesian Submarine retrieval will have Chinese assistance to haul it from the seabed at 838m, which requires deep-sea vehicles to accomplish that. One of the biggest questions is how did it happen.

Indonesian Submarine retrieval is underway to know its circumstances

The Indonesian navy experts say an internal solitary wave caused the sinking of KRI Nanggala 402 and the losses of its 53 crew, which is known to occur in the seas around Bali, which is a normal phenomenon. It seems the sub was caught unaware by the deep-sea wave, report RNZ.

It sunk to an extreme depth of 838 meters, well beyond rescuers' reach. Getting the subs remains will need specialist diving equipment, citing the BBC.

Authorities said there was no hope many would survive as crew members' personal effects floated up and the oxygen level onboard slowly expired.

Many theories have been proposed as to what happened. Still, experts now suggest that an underwater wave - which can exert a sizeable vertical pull under the sea surface - arose in the Bali Sea about time the submarine disappeared last Wednesday morning.

How is a solitary wave under the sea?

The Lombok Strait, which connects the islands of Bali and Lombok, is notorious for producing these strong internal waves on a virtually fortnightly schedule.

Strong tidal currents, a rough ocean floor, and the exchange of water across two channels - one shallow and then one deep, "tend to combine about every 14 days to generate an exceptionally strong tidal movement," said NASA. Accomplishing the Indonesian Submarine retrieval will be dangerous in such a condition.

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This natural phenomenon, remarked Indonesian navy officials, is a more likely explanation for the submarine disaster than other hypotheses proposed in recent days.

On the ocean's surface, internal waves are almost invisible. They may, nevertheless, reach incredible depths underneath the sea.

According to Rear Admiral Muhammad Ali, an internal wave, a former commander of KRI Nanggala 402, "a strong current that really can drag the sub vertically, forcing it to sink faster and crash into the sea floor."

He added that it must have been natural conditions that crushed the submarine—mentioning that vertical wave was forming north of Bail at that time.

According to navy officials, images from Japan's Himawari 8 satellite and Europe's Sentinel satellite showed large underwater waves from around the time the KRI Nanggala 402 sank.
"It moved up from the bottom to the north, and there's a depression between two mountains," Commander of the Indonesian Navy Command and Staff School Rear Admiral Iwan Isnurwanto added.

"The wave was about two nautical miles [in speed], and the volume of water was between two and four million cubic litres."Admiral Iwan explained an internal wave could render the crew helpless in the face of nature; he said it could be what happened.

Three Chinese vessels have entered Indonesian waters to help in the retrieval of a submarine that plummeted and split apart last month, killing 53 people on board, told by an official. One of the vessels has a manned submersible capable of sinking to 10,000 meters (32,800 feet).

Indonesian navy officials said the Indonesian Submarine retrieval would need all the help, even the Chinese, to get it from the seafloor, noted SCMP.

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