Two of them Ran Into Each Other

Nuclear submarine production numbers since the 1950s have been revealed, and while they are significant, they are not astounding. It's not like they can't avoid one another because the oceans are so congested with them. They do, however, find it difficult to avoid one another. It got to the point in 2009 where two of them actually collided.

Such a collision can be seen as a success on the one hand. It indicates that the subs are so well built and stealthy that they are practically undetectable under the water. Le Triomphant, a French ship, and Britain's HMS Vanguard collided. None of the crew members of either vessel were hurt because it was moving at a slow speed. However, given that these were two nuclear ships and that the Vanguard had 48 nuclear warheads on board, it did cause a lot of people to become tense. Everyone was nervous because they were worried that a catastrophic nuclear explosion may happen.

So how did the mishap occur? Never was it completely explained. Both subs may have spotted each other if they had been using active sonar, but since subs won't use sonar when hiding their location, it appears to be merely a remarkable coincidence.