A frightening invisible natural force is believed to have dragged an Indonesian submarine deep into Balinese waters last week.
The KRI Nanggala 402 lost contact after it submerged during a routine training exercise in the Bali Sea last Wednesday.
After a frantic five-day search, the attack vessel was found by rescuers cracked apart on the sea bed at a depth of 838m. All 53 crew members had died.
After objects from inside the submarine were found during the search, questions began to arise over how the manned submarine could disappear so quickly.
How did the KRI Nanggala 402 disappear?
Officials from the Indonesian military now suspect the submarine was hit by an internal solitary wave, a powerful force generated when a volume of water is pushed through a relatively smaller passageway.
Officials said the density of waters off the coast of Bali and in the nearby Lombok Strait likely triggered a "massive moment" of force, with enough downward momentum to suck the submarine downwards within moments, according to Nikkei Asia.
What are internal waves?
Internal solitary waves are powerful, hard to detect currents that cause a major threat to submarines and can put large stress on offshore oil rigs.
Satellite imagery captured by NASA in 2016 shows an internal solitary wave off the Lombok Strait.
The space agency explained internal waves occur when the "interface between layers is disturbed, such as when tidal flow passes over rough ocean floors, ridges, or other obstacles".
Because the Lombok Strait is a relatively narrow passage between the Pacific and Indian Oceans, the tides develop a "complex rhythm" but "tend to combine about every 14 days to create an exceptionally strong tidal flow".
The combination of ocean topography, strong currents and moving water between the two oceans makes the area famous for its generation of intensive internal waves.
The waves are difficult to photograph, and cause only minor ripples on the surface of the ocean, despite their potential to create a strong underwater drag.
'There was nothing that they could do'
Iwan Isnurwanto, commander of the Navy Staff and Command School said images of a suspected internal solitary wave in the area where the submarine disappeared were 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," Isnurwanto said at a news briefing at navy headquarters in Jakarta.
"It likely angled (downward), causing all the crew members to roll down (to the bottom of the vessel).
"We have to do further investigation, but that is most likely what happened."
Other theories on why the sub sunk
Before the KRI Nanggala sunk, reports suggested the attack vessel may have experienced a power outage.
The theory came from a provisional analysis submitted by the Navy Information Service, and said the submarine was at risk of a possible black out during static diving. The analysis warned the submarine could fall while at a depth of 600-700m.
It was suggested the ship needed an emergency button to counter the issue.
A former general TB Hasanuddin also suggested a refurbishment and retrofit of the submarine, completed in 2012 in South Korea, may have been completed improperly.
He said the same year the retrofit was completed there'd been a failure with the test firing of the torpedo system in the ship, killing three people.
"I suspect that in the results of this repair there are things or construction that is not right so that the KRI Nanggala-402 sank. This is very unfortunate," he said.
The vessel was first built in 1978 and had its last refit in 2012, when its depth was increased to 250m.
During the search, crews had discovered an oil spill, and several objects including prayer mats, and pieces of the vessel's torpedo tube, hours after the KRI Nanggala lost contact. The findings sparked immediate concern for the welfare of those on-board.
It's believed the water pressure caused the submarine to split, through which some of the items escaped.
The air supply had been expected to expire at 3am last Sunday.
Search ships received magnetic signals from the submarine on Sunday morning, and confirmation the vessel was on the sea floor came soon after.
Joko Widodo, the Indonesian president, said the nation was shocked by its first submarine disaster, and the government "expresses our deep condolences, especially to the family of the crew members".
Finding the KRI Nanggala 402 ended a panicked five day search that involved nations surrounding Indonesia, including the US and Australia.
One rescue vessel, the MV Swift Rescue, was capable of undersea retrieval of people, had there been any survivors, the Times News Service reports.
The vessel's crewed submersible is capable of carrying up to 17 people to the surface and has decompression chambers and medical centres.
The KRI Nanggala 402 was one of five submarines in the Indonesian fleet and was refitted in South Korea in 2012.
– With wires