Losharik (Russian: Лошарик) is the nickname of a very special Russian nuclear submarine with multi-spherical hull construction (from this derives the nickname, taken from a Russian cartoon character, a toy horse consisting of small spheres).
Very few informations are available: this nuclear submarine is believed to be able to operate at a depth of many 1000s metres due to the unique spherical construction elements. The exact operational depth is unknown but it is known that this sub operated at about 2000-2500m depth in the Arctic during 2012.
Moreover a Delta IV SSBN has been modifified as deep-sea station, removing the missile tubes section and replacing it with another longer section (from K411) in order to host equipments and to "hold" Losharik...
Here are the two units.