Turns out this was no average submarine:
14 Sailors Die on Secretive Russian Nuclear Submarine
(excerpt)
The boat was identified by Russian-language news service RBC as Losharik (AS-12), a nuclear-powered submarine that is widely believed to be a key asset for the Russian Main Directorate of Deep-Sea Research, also known as GUGI. GUGI develops and operates a fleet of specialized submarines that Russia uses for deep sea and Moscow’s most covert operations.
Losharik is among the most mysterious of the closely guarded fleet. The about 2000-ton boat can travel slung under the belly of a specially modified Delta III nuclear ballistic missile submarine, according to open source intelligence analyst).
bkmk.