At least we are pretty sure. Read on.
In 1981, A British Submarine Smashed Into a Russian Sub (Armed with Nuclear Weapons)
On May 23, 1981 the Soviet submarine K-211 Petropavlovsk cruised quietly at nine knots, one hundred and fifty feet below the surface of the Arctic Barents Sea. The huge 155-meter-long Delta III (or Kalmar)-class submarine was distinguished by the large boxy compartment on its spine which accommodated the towering launch tubes for sixteen R-29R ballistic missiles, each carrying three independent nuclear warheads.
K-211’s mission was hair-raisingly straightforward: to cruise undetected for weeks or months at a time, awaiting only the signal that a nuclear war had broken out to unleash its apocalyptic payload from underwater on Western cities and military bases up to four thousand miles away.
British and American nuclear-power attack submarines (SSNs), or “hunter-killers,” were routinely dispatched to detect Soviet ballistic missiles subs (SSBNs) leaving from base to discreetly stalk them. The quieter SSNs also awaited only a signal of war, an event in which they would attempt to torpedo the Soviet subs before they could unleash their city-destroying weapons.
Mindful of this threat, at half past seven that evening K-211’s commander halted his sub and pivoted it around so that its MGK-400 Rubikon bow sonar array could attempt to pick up any submarines sneaking behind it in the ‘blind spot’ of its wake—a maneuver known as “clearing the baffles.” However, the SSBN’s hydrophones did not report any contact.