The submarine at the centre of the fire that killed at least 14 crew members in Arctic waters on Monday is the most mysterious and closely guarded of Russia's fleet.
Russian officials have offered little on the circumstances surrounding the incident but the presence of several top ranking military officials on board suggests it was on a mission of great significance.
President Vladimir Putin said seven of the dead were Captain First Rank officers — the most senior staff officers in the Russian navy. Two were recipients of the Hero of Russia award which is only handed out by the president.
According to the defence ministry, the victims died from smoke and toxic fumes generated by a fire on board a "scientific research deep-sea submersible" mapping the sea floor.
It is believed the vessel in question is none other than the "Losharik", an AS-12 nuclear powered mini spy sub which can operate at depths of up to 6000m — ten times that of a regular sub.
US authorities have previously accused Russia of attempting to intercept or sabotage communications via the Losharik's extraordinary deep sea capabilities to tamper with or even sever underwater cables.
The sub was nicknamed in honour of an animated toy horse of the same name made from multi-coloured bubble-like orbs. While very little is known about the Losharik's design, military experts believe the sub contains seven bubble shaped compartments.
"The use of these spherical fragments in the design create conditions so that the ship can safely dive deeply. This is the origin of the nickname," specialist website Covert Shores, which features several plans of the Losharik.
Of course, a horse with hidden compartments has many other connotations — in war and computer malware.
The Russian military has never released a photograph of the Losharik but dozens of diagrams and artist's impressions purporting to be of the model can be found on amateur and commercial military websites.
Bizarrely, the only known image of the Losharik accidentally appeared in the Russian edition of Top Gear Magazine in a feature about a test car rally on a Mercedes Benz GL 450 car to Akhangelsk.
"On the shores of the White Sea, motorists of the magazine staged a photo session, at the same time passing a submarine peacefully passing along the coast — without being aware of what they were shooting," the Center for Analysis of Strategies and Technologies wrote in its unofficial blog.
The fire on board the Losharik is believed to have taken place on Russia's northern shore in the Barents Sea at around 8.30pm on Monday.
/arc-anglerfish-syd-prod-nzme.s3.amazonaws.com/public/6QBH2MHXRNGE7LADZ7STXPBO6Y.jpg)
Norway's radiation authority revealed that Russia had informed it that there had been a gas explosion on board the sub — a claim denied by Moscow.
"There has been a gas explosion, confirmed by the Russian authorities," Norwegian Radiation Protection Authority (NRPA) Per Strand told the AFP.
"We are waiting for information from the Russian side about whether there was a reactor on board the submarine," he said, adding his agency had not detected an increase in radiation levels.
Russia's defence ministry denied having said any such thing to Norway, declaring "there were no notifications sent to the Norwegian side regarding the Russian science research deep water apparatus".
President Vladimir Putin ordered a full investigation into what he deemed a "tragedy" in the country's far north, the latest in a string of disasters and accidents to hit Russia's navy.
A translation of a Ministry of Defence statement issued by the state-run TASS news service said 14 sailors died "in Russian territorial waters as a result of inhaling combustion products aboard a research submersible vehicle designated for studying the seafloor and the bottom of the World Ocean in the interests of the Russian Navy".
It said the fire "broke out during (the taking of) bathymetric measurements" before it was extinguished by the "self-sacrificing" actions of surviving crew.
The vessel has since been returned to a military base in the northern city of Severomorsk on the Kola Peninsula above the Arctic Circle.
It is the same base which serviced the doomed Kursk submarine, which sank in the icy Barents Sea in August 2000, after a torpedo exploded, denonating all the others and killing all 118 on board.
The disaster happened in Mr Putin's first term as president and he was severely criticised for failing to cut short his holiday to address the tragedy.
This time the leader did not dawdle, cancelling a scheduled appearance to summon Defence Minister Sergei Shoigu for an immediate briefing.
"It is a big loss for the navy, and for the army as a whole," Mr Putin said of the accident, AFP reported.
"It is not an ordinary vessel, as we know, it's a scientific-research vessel, its crew is highly professional"
The fire is the latest in a series of accidents to befall the Russian military.
In August 2000, the Kursk submarine sank in the Barents Sea, killing all 118 on board.
An inquiry found that a torpedo had exploded, detonating all the others.
In 2008, 20 people, including three naval officers and 17 civilians, were killed by poison gas after a vessel's fire-extinguishing system was accidentally activated during trials in the Sea of Japan.
In 2011, one of Russia's biggest nuclear submarines caught fire while undergoing repairs in dock in the northern Murmansk region.
Later, it was reported the sub was armed with long-range nuclear missiles when it caught fire.
