The Paltus Class submarine is a Russian special purpose mini-submarine of project 1851.1. Two boats were completed "AS-21" and "AS-35" as a follow up of the single "X-Ray" class boat "AS-23" (Project 1851). They are both part of the 29th special submarine squadron at Olenya Guba.
References
Paltus-class submarine Wikipedia(Text) CC BY-SA