Losharik russian submarine
Reported picture of Losharik - Reports say that the Losharik, also called the AS-12, was on a research mission to measure sea depths in the Barents Sea within Russian territorial waters.
When the story first broke yesterday that fourteen seaman aboard a Russian submarine died when a fire broke out while in Russian territorial waters in the Arctic region, there were conflicting accounts over the nature of the underwater vessel, specifically, whether it was a deep-sea research submersible or whether it was a nuclear powered submarine. Speculation was fueled further when the fact that many among the deceased were high ranking naval officers was revealed — unusual for a mere underwater "research" vessel.

Russian President Putin has now confirmed it was indeed a nuclear-powered submarine after officials declined to reveal even the most basic details, citing its "secret mission". As CBS reports:
Putin met Thursday with Defense Minister Sergei Shoigu, who had returned from the Navy's Arctic base of Severomorsk, and asked him about the submersible's nuclear reactor. Shoigu said the reactor is "completely isolated now" and it is "in full working order."
The blaze has been identified as starting in the sub's battery compartment and spreading from there, causing 14 crewmen to succumb to smoke inhalation, while an undisclosed number survived. Norwegian officials had also previously reported that they have discovered no unusual levels of radiation near the accident.

Russian officials have not confirmed the precise name and type of vessel; however, Russian media reports appear to have identified it as follows:
The ministry didn't name the vessel, but Russian media reported it was the country's most secret submersible, a nuclear-powered research submarine called the Losharik intended for sensitive missions at great depths.
Reports say that the Losharik, also called the AS-12, was on a research mission to measure sea depths in the Barents Sea within Russian territorial waters. It's missions and specs are so tightly guarded only select military members with the highest clearance have access to it.

But given that mainstream western media can't waste a Russian submarine tragedy without adding wild accusations of sinister wrongdoing over what the "sensitive mission" could have been about, less than 24 hours later we have the "Putin's cutting the world's internet" claims again:
BBC Monitoring described US officials as saying the AS-12 is designed to cut undersea cables that keep the world's internet running.
It's an accusation that's been floating around the internet for years. All the way back in 2015 a NYT story cited the Russian underwater cable threat as "raising concerns among some American military and intelligence officials that the Russians might be planning to attack those lines in times of tension or conflict."
map global undersea internet cables
Map showing location of global undersea internet cables via Telegeography
Putin submarine, Putin internet cables
© AFP/GettyRussian submarine accident? Cue Putin cutting underwater internet cables story again.
Of course, such unconstrained and rampant speculation works better in the complete absence of any level of evidence, and when the Russian military cites a "top secret project".

Kremlin spokesman Dmitry Peskov told reporters Wednesday when pressed about the precise nature of the Losharik's activities: "It belongs to the highest level of classified data, so it is absolutely normal for it not to be disclosed."