The tragic news of the Indonesian submarine KRI Nanggala being declared sunk on 25th April 2021 by the Indonesian Navy is still fresh in the minds of the masses in the form of the heart-wrenching video of 53 sailors aboard singing the farewell song.
But how did the submarine KRI Nanggala 402 sink?
The best plausible hypothesis which can explain the sudden disappearance of the submarine in the early hours of 21st April and the condition of the wreckage being found at a depth of 2700ft on the ocean bed in three separate pieces off the coast of Bali is "internal waves".
The term "internal waves" might sound very new to many but each one of us has undoubtedly witnessed the occurrence of this phenomenon in one form or the other. It prevails everywhere in nature from the atmosphere to the oceans. The turbulence that a flight faces in the atmosphere or the clouds in the form of thin bands are all the result of propagating internal waves.
Internal waves in the atmosphere are produced as a result of high-speed winds traveling uphill in steep mountain ranges against gravity after which they accelerate down the hill but this time in an up and down motion of oscillating fashion. To understand this phenomenon more simply the downward motion of the winds can be equated to the motion of a ball thrown down a staircase bouncing off at each step.
Similarly, the generation of internal waves in oceans is the result of strong ocean currents or channels propagating across steep hilly terrains of the ocean bed. The resulting internal waves intensify with increasing strengths of ocean currents. The internal waves are majorly the reason that ocean currents persist over large distances.
Submarine KRI Nanggala 402 went missing off the coast of Bali where the topography of the seafloor is incredibly complex with deep trenches, troughs, and basins creating specific upwellings like the barrier near the Lesser Sunda Islands. The region also experiences strong ocean currents throughout the year like the Indonesian throughflow (ITF) making it the region prone to strong internal waves.
One such extreme type of internal wave termed “internal solitary wave” is produced by strong ocean currents where the consolidation of entire wave energy usually divided over several oscillations occurs, resulting in one single large oscillation, which in few cases is even miles long, traveling at speed greater than five knots.
KRI Nanggala submarine was a cakra-class attack submarine whose maximum operating depth was 250 meters and had a crush depth of 500 meters which means beyond this depth the hull cannot recover from damages caused due to the increasing pressure.
Under normal circumstances KRI Nanggala 402 operational at a depth less than 250 meters where typically the temperature difference between surface water and water at deeper sea levels is relatively high which can further intensify the internal solitary waves increasing the speed to fatal levels of more than 180ft/per hour.
In the case of submarine KRI Nanggala being hit by a strong internal solitary wave of such magnitude at a particular relative position from where it may have been led downwards with a sudden increase in its depth, way beyond its crush depth of 500meters, ultimately may have led to hull failure and sinking with eventual rupture causing the submarine to split in three-part taking the toll of all 53 onboard.
Investigations regarding the primary cause of the disappearance and sinking along with reasons to back the unsuccessful attempt to execute emergency procedures are still underway, but as suggested by Admiral Margono the initial analysis indicates "natural factors" as the reason behind the accident and the hypothesis of "internal solitary waves" seems to prove it right.