Although French and British navy officials announced on February 16th that two of their SSBNs, the French Le Triomphant and the British Vanguard, had collided with each other on February 4th, no more details were given. While the British have remained tight lipped, French officials have relented under media pressure, and made several interesting revelations (some openly, others "off the record.")
First, the February 6th French announcement that the Le Triomphant had collided with some unknown underwater object, causing some damage to its sonar dome, was not a deliberate deception. At the time, the crew did not know what they had hit, and assumed that it was a cargo container, floating underwater after having fallen off a ship. It's a common enough occurrence in bad weather. Nothing else seemed to make sense. Ten days later, after the British announced that their SSBN had also collided with something underwater, they compared notes with the French, and concluded that the two boats had run into each other. No one has revealed if any analysis was done on materials from the hull of each boat left on the other after the collision. This would be definitive proof of a collision. This analysis may still be underway.
It appears that the bow (front) of the Le Triomphant scrapped along the side of the HMS Vanguard. The French apparently believe that both boats were so quiet (which is how SSBNs are designed to operate), that neither boat detected the other one, even after the collision. The British have not released any information on what went on inside their boat before, during or after the collision. The French revelations indicate that onboard the Le Triomphant, there was bumping and scraping, then silence. There was probably a damage assessment drill, which came up clean, except for some damage to the sonar dome (which meant the sub was even less able to hear any noises coming out of the Vanguard). The Le Triomphant then proceeded on its way, only able to guess at what it had just hit.
The French also revealed that they do exchange location knowledge with NATO navies with regard to where their attack submarines are operating under water. But they consider the SSBNs one of their strategic weapons and thus subject to a higher degree of secrecy. For decades, it's been understood that sharing information with all your NATO partners increases the chances of the secrets getting out. So even if France rejoins NATO, they will continue to keep the locations of their SSBNs to themselves. Since only one or two of these SSBN boats are at sea at any time, it was believed that collisions would never be a problem. The odds of an underwater collisions were considered so high as to be virtually impossible.
It will be interesting to hear what the British have to say, eventually.