Russia's submarine fleet is more active than it's been in decades in a strategically vital body of water — and US military planners can't seem to figure out why.

US Navy Vice Admiral Clive Johnstone, also head of NATO's maritime command, told IHS Jane's that the US has seen "more activity from Russian submarines than we've seen since the days of the Cold War" in the North Atlantic.

While Johnstone didn't specify which parts of the North Atlantic have seen this uptick, nearly the entirety of the area between North America, the Western European coastline, and the bottom of the Arctic Circle is vital to US and allied security.

And Russia isn't just sending submarines in volume.

It's also sending vessels with what Johnstone describes as unprecedented capabilities within the Russian fleet.

Thanks to "an extraordinary investment path not mirrored by the West," Russian submarines now "have longer ranges, they have better systems, they're freer to operate."

Meanwhile, the crews have demonstrated "a rise in professionalism and ability to operate their boats that we haven't seen before."

US Navy crew participating in NATO's Dynamic Mongoose antisubmarine exercise in the North Sea. Reuters

Johnstone admits that the US has little idea what's behind the increase in Russian submarine activity, which wouldn't automatically be worrying "if we knew what the game plans were or we knew why they were deploying or what they were doing."

Instead, Johnstone says, "we don't understand what the strategic and operational objectives are of the Russian state," with Russia's purposes remaining "obscure and ... shrouded in other activity which makes us nervous, and makes nations nervous."

Thomas Gibbons-Neff of The Washington Post suggests that the submarines may be part of a bugging operation, noting that "the United States has monitored Russian submarines and surface ships patrolling around undersea fiber optic cables."

Mike Nudelman/Business Insider

As Gibbons-Neff says, "tapping underwater communication lines is an old Cold War tactic revolutionized by the US Navy in a series of spy missions that began in 1970."

An increased deployment could be connected to Russia's Arctic pivot — Moscow's construction of military bases along the country's northern rim as part of a policy aimed at claiming strategic space as the Arctic gains in importance as a commercial corridor.

It could have to do with Russian power projection in northern Europe, an objective that actually appears to predate the outbreak of the Ukraine crisis and resulting tensions between Russia and Europe. According to a recent NATO report, Russia carried out a mock nuclear attack on Sweden in March 2013.

Meanwhile, Russia conducts frequent aerial and naval incursions along its western and eastern borders, a tactic that demonstrates the country's military operability and tests the resolve and overall readiness of neighboring states. In October 2014, Sweden launched its largest military operation since the Cold War in response to a reported incursion of a Russian submarine in the country's territorial waters.

While it certainly matters why Russia is increasing its undersea presence in a zone of US strategic importance, the Kremlin's overall goals might be an afterthought in the event of an unintended escalation between US and Russian ships — the chances of which increase as long as Russia continues to engage in large-scale deployments that puzzle even top American naval officers.