The Krasnodar, a Russian attack submarine, left the coast of Libya in late May, headed east across the Mediterranean, then slipped undersea, quiet as a mouse. Then, it fired a volley of cruise missiles into Syria.

In the days that followed, the diesel-electric sub was pursued by the aircraft carrier USS George H.W. Bush, its five accompanying warships, MH-60R Seahawk helicopters and P-8 Poseidon anti-sub jets flying out of Italy.

The U.S. and its allies had set out to track the Krasnodar as it moved to its new home in the Black Sea. The missile attack upended what had been a routine voyage, and prompted one of the first U.S. efforts to track a Russian sub during combat since the Cold War. Over the next weeks, the sub at points eluded detection in a sea hunt that tested the readiness of Western allies for a new era in naval warfare.

An unexpected resurgence in Russian submarine development, which deteriorated after the breakup of the Soviet Union, has reignited the undersea rivalry of the Cold War, when both sides deployed fleets of attack subs to hunt for rival submarines carrying nuclear-armed ballistic missiles.

When underwater, enemy submarines are heard, not seen—and Russia brags that its new subs are the world’s quietest. The Krasnodar is wrapped in echo-absorbing skin to evade sonar; its propulsion system is mounted on noise-cutting dampers; rechargeable batteries drive it in near silence, leaving little for sub hunters to hear. “The Black Hole,” U.S. allies call it.