In what could have become a national incident, a Royal Navy submarine had a near miss with a car-carrying ferry in the Irish Sea last November, according to reports that recently emerged.

These reports state the submarine was traveling at periscope depth in the waterway that lies between England and Ireland when it came dangerously close to colliding with the large ferry.

“In November, we were notified of a close-quarters incident between the ro-ro [roll-on/roll-off –ed.] ferry Stena Superfast VII and a submarine operating at periscope depth,” said a spokesman for the U.K.’s Marine Accident Investigation Branch (MAIB). “We have carried out a preliminary assessment of the evidence in this case and the Chief Inspector of Marine Accidents has decided to open a safety investigation. The investigation is being conducted with the full co-operation of the Royal Navy. A report will be published when our investigation has concluded.”

Thus far, the Royal Navy hasn’t indicated which of its ten operational submarines was involved in the incident. All their submarines are nuclear powered, with six falling under the category of “fleet submarines” and the other four serving as “ballistic submarines” tasked with serving as the sea-based leg of the U.K.’s nuclear deterrent apparatus. At least one of these Vanguard-class submarines–armed with 40 nuclear warheads and eight Trident II D5 missiles–is continuously at sea and on patrol to ensure the nation’s ability to mount a nuclear response if ever an attack were to occur.

It stands to reason this close call may have involved a submarine armed with nuclear weapons, although it remains just as likely that one of the U.K.’s Trafalgar or Astute class subs, traditionally armed only with conventional ordnance, may have been the sub in question.

“We can confirm the sighting of a Royal Navy submarine between Belfast and Stranraer on 6 November 2018. We are co-operating with the MAIB’s investigation,” a Royal Navy spokesperson told the press.

The Stena Superfast VII is a 660-foot vessel can carry as many as 1,300 passengers and 600 cars, making it a rather large target to miss for the Royal Navy submarine just beneath the surface of the water.

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Royal Navy nuclear submarine nearly collided with a ferry in the Irish Sea
Stena Line

Had a collision occurred, it would be far from the Royal Navy’s first embarrassing submarine mishap. In fact, despite employing only 10 active submarines in the force, three were involved in embarrassing incidents within the last decade.

In 2009, the ballistic missile sub H.M.S. Vanguard, namesake of its class, collided with the French submarine Le Triomphant in the Atlantic. Seven years later, the H.M.S. Ambush collided with a civilian vessel off the coast of Spain. In 2015, another submerged Royal Navy submarine snared the nets of an Irish fishing boat and nearly dragged it under water before the fishermen cut the lines.