Russian submarines have become increasing assertive in the Atlantic, and the Pentagon finds itself upping its game to try to maintain its supremacy.
"We were operating in places where we didn't have to rely on an adversary being there to challenge us. That's changing," U.S. Navy Commodore Ollie Lewis told CNN as part of an exclusive visit to the USS Missouri nuclear submarine, part of a squadron of 10 Atlantic-based submarines that Lewis commands.
"So we're back to the point now where we have to consider there is an adversary ready to challenge us in the undersea domain and that undersea superiority is not guaranteed," Lewis said.
The $2 billion Virginia-class nuclear-powered submarine -- which is 377 feet long, weighs 7,800 tons and can travel over 25 knots -- is considered the most advanced submarine in the world.
CNN joined the crew for exercises this week near the Florida coast. But it has often found itself at the front lines of what many are billing as a new Cold War. When Russia annexed Crimea and launched military action in Syria, the Missouri was deployed nearby. Similarly, when a Russian sub turned up off the coast of Florida in 2012, the Missouri was called into action to track it.
The submarine can launch torpedoes at other submarines and at ships. It can also launch missiles at ground targets. It gathers intelligence. It could also deploy Navy SEAL units for special operations.
The Virginia class is "the Swiss Army knife of submarines," according to Andrew Hunter of the Center for Strategic and International Studies.
Unlike other submarines, such as ballistic missile subs, which are geared towards one specific mission, Hunter said submarines such as the Missouri are capable of carrying out a wide range of missions, including intelligence and sea control.
But it will soon be challenged by a new series of Russian submarines, the Yasen class, the first of which is undergoing weapons trials.
Michael Kofman, an expert on the Russian military and fellow at the Wilson Center, a Washington-based think tank, told CNN that the Yasen class was "the quietest submarine operated by an opponent," thereby making it difficult to keep tabs on.
"The Navy is not really sure it can track it," Kofman said. He later clarified that he was speaking generally about the challenge of tracking the sub and not commenting specifically about the U.S. Navy's capabilities.
As Magnus Nordenman, a Russian military expert at the Atlantic Council put it, "For Russia the submarine is the crown jewel of their fleet, much in the way the aircraft carrier is the crown jewel of the U.S. Navy."
Even before the new vessel is christened, the Cold War power is stepping up its underwater activities.
The U.S. Navy told CNN in April that Russian submarine activity was reaching levels unseen for decades.
The USS Missouri's commander, Fraser Hudson, assessed that the renewed Russian activity is not just "a political statement."
He told CNN Tuesday that he thinks the Russians are seeking to gain experience in case hostilities were ever to break out between it and the United States.
"Honestly, I think it's operational experience. You maintain the experience in those (areas of responsibility) so that if anything were ever to happen, they have experience," he said.
After pausing submarine activity in the wake of the 2000 Kursk disaster that resulted in the deaths of 118 Russian sailors, Moscow has increased patrols of the Atlantic, particularly in the critical Greenland-Iceland-UK triangle, which would become a vital sea lane were the U.S. ever required to reinforce troops in Europe.
Hudson told CNN that "there has been an increase in the last 10 years" of Russian subs operating near U.S. waters.
Experts, while noting that the Russian submarine fleet is much smaller that the Soviet Union's was during the 1980s, have said that the U.S. Navy was particularly concerned about the renewed Russian actions due to the fact that the U.S. and allied anti-submarine warfare capabilities have atrophied since the end of the Cold War.
"The U.S. has sort of been neglecting its Anti-Submarine Warfare operations because there really hasn't been a need for it," Dimitry Gorenburg, a research scientist with the Center for Naval Analyses, told CNN.
In the face of this remerging threat, CNN observed the Missouri's 135-member crew repeatedly train for anti-submarine warfare.
"Submarines are a dangerous business. There is always tension wherever you go because we operate in a challenging environment," Hudson said.