Russia's submarine fleet is one of the most capable in the world, perhaps second only to that of the US. And it is only getting better. Although the subs are holdovers from the Cold War, Russia has been steadily investing in and modernizing its fleet in capabilities and range. Russian President Vladimir Putin has grand plans to modernize with the purchase of additional subs and the development and acquisition of new models.

And Putin's plan has so far worked. Russian submarines are active again in the North Atlantic at levels similar to those during the Cold War. And their training and professionalism are suddenly drawing the concern of NATO. In response to these actions, the US is once again opening a Cold War-era sub-hunting base in Iceland.

Below is an infographic by St. Petersburg, Russia-based designer Anton Egorov depicting the submarines that Russia currently operates, along with their maximum depth:

Anton Egorov

Russia's sub fleet is divided into three broad categories: diesel-electric powered, nuclear-powered ballistic missile, and nuclear-powered attack. Each variation has its own unique purpose and is subdivided into varying models.

The country's diesel-electric fleet is the least technologically advanced segment and the cheapest to buy and maintain. These submarines, which are smaller and slower and have a shorter range than their nuclear counterparts, are limited in their total operational depth and are used for attacking surface ships and merchant vessels.

Russia plans on adding an additional six Kilo-class subs to the Black Sea Fleet, along with 14 to 18 diesel-electric submarines similar to Lada-class subs over the next 15 years.

Borei-class submarine. Wiki Commons

Nuclear-powered ballistic subs form the nuclear-deterrent backbone of the Russian fleet. These subs are faster than diesel-electric submarines, larger, and can dive to significantly deeper depths. These subs carry ballistic missiles with nuclear warheads.

The final class in the fleet are nuclear-powered attack subs. These submarines are generally the fastest in the fleet and are usually slightly smaller than ballistic-missile subs, although they can usually operate at lower depths. These submarines are used to hunt ballistic-missile subs and enemy ships. In some cases, the subs can be armed with vertical-launch tubes to target land-based objects.

Russia plans to replace its Delta III- and Delta IV-class submarines with Borei II subs in the coming years. The Oscar II class will be replaced with the new Yasen class after 2020. Of course, continued Russian modernization of its fleet may face some serious issues. Sanctions and continuing low oil prices are seriously cutting into Russia's budget to the point that Moscow might sell off portions of its state-owned oil companies.