Earlier this month, a Russian ballistic missile submarine (SSBN) called Podmoskovie slipped out of its pier at Severodvinsk for the first time in 16 years.

But BS-64 Podmoskovie—which was commissioned in 1986 as a Project 667BDRM Delfin-class (NATO: Delta IV) SSBN designated K-64—is no ordinary boomer. Over the course of nearly two decades, the massive submarine was modified to conduct special missions. But exactly what those missions might be remains somewhat of a mystery.

Podmoskovie was photographed leaving the shipyard for contractor sea trials on Oct. 22 by Oleg Kuleshov, who writes for the BMPD blog—a product of the Moscow-based Centre for the Analysis of Strategies and Technologies.

Podmoskovie and her sister BS-136 Orenburg—a former Delta III SSBN—are roughly analogous to the U.S. Navy’s secretive USS Jimmy Carter (SSN-23)—which is a highly modified Seawolf-class boat. Carter is roughly 100ft longer that her two Seawolf-class sisters with the addition of a Multi-Mission Platform (MMP), which allows the submarine to launch and recovery of various unmanned vehicles and support special operations forces. Podmoskovie is thought to be similar in concept—but the Russians are not exactly keen on sharing those details for obvious reasons.

What is known about Podmoskovie is that the massive vessel entered the shipyard in 1999 under the Russian Ministry of Defense’s Project 09787—which ostensibly performs deep-sea research. By 2002, the boat had its missile tubes removed and the special compartments similar to those on Orenburg were installed. Indeed, externally, Podmoskovie looks very different from a standard Project 667BDRM boat aft of the sail and she appears to have had her hull lengthened.

Podmoskovie is able to launch and recover unmanned underwater vehicles, which dock on top of the submarine where the missiles used to be located. One such unmanned submarine is the Klavesin-1R—which was developed by Russia’s Institute of Marine Technology. The unmanned submarine is able to dive to depths as great as 6000m—or nearly 20,000ft. The unmanned vessels are equipped with a variety of high and low frequency sonars.

Podmoskovie is also thought to be able to host the secretive AS-12 Losharik—a nuclear-powered mini-submarine designed for intelligence and special operations missions at extreme depths—perhaps as great as 20,000ft. While very little is known about Losharik, the vessel is believed to be tasked with tapping undersea cables among its various other missions.

Additionally, Podmoskovie will almost certainly host the Project 1851 Paltus and Project 1910 Kashalot nuclear-powered special operations mini-submarines. Like Losharik, the Paltus and Kashalot are thought to have both a research and military role. However, due to the level of secrecy surrounding these programs, there is very little information available about these vessels.

It will take some time for Podmoskovie to complete its various sea trials, but all indications suggest that she will be a capable addition to the Kremlin’s arsenal when she returns to operational service. Podmoskovie is expected to joint the Russian Northern Fleet’s 29th Submarine Brigade when she rejoins the fleet.

Dave Majumdar is the defense editor for The National Interest. You can follow him on Twitter: @davemajumdar.