On February 16th, the French and British navies confirmed that two of their SSBNs, the French Le Triomphant and the British Vanguard, had collided with each other on February 4th. The Vanguard was on patrol, while the Le Triomphant was returning to port (Brest) from a patrol. It appears that the Le Triomphant hit the Vanguard, while running in a parallel collision course. Both France and Britain have long maintained a force of four SSBNs each.
Earlier, on February 6th, France had announced that The Le Triomphant had collided with some unknown underwater object, causing some damage to its sonar dome. Ten days later, the French admitted that they had lied, trying to cover up what really happened. By then, civilians had seen that the Vanguard had dents and scrapes along its side, indicating that the some other vessel had made contact with a long portion of the Vanguard's hull. This was visible on February 14th, as the Vanguard returned, early, to its base in Scotland.
How two SSBNs could bump into each other in such a large ocean, is to be the subject of a joint investigation by the two nations. There are several plausible reasons for such a collision taking place.
First, both France and Britain have their SSBN patrol areas in the same patch of ocean. That's because the range of their missiles, and the location of potential targets (Russia, the only nation with nukes aimed at Europe) means there's only a small area of the eastern Atlantic where these patrols are going to take place. So while the two nations SSBNs are still operating, underwater, in a large bit of the Atlantic, it's not as big as you might think.
Second, SSBNs operate as quietly as possible. They use passive (it just listens) sonar and move slowly (about 10 kilometers an hour). Just how quiet SSBNs are is considered classified information, as is the possibility that two of them could be very near each other, and be undetectable to each other. It may now be revealed if the U.S., or anyone else, ever sought to discover if this was possible. Certainly, the British-French investigation of the SSBN incident will probably make it clear if these boats are quiet enough underwater to be invisible to each other. Note that this invisibility may only happen because of special conditions underwater (different temperature or salinity layers of water, which channel the sound away from layers above and below). Since this is all very sensitive material, the full results of the investigation may not be released for decades.
Third, there is a lack of cooperation between the French and other navies. NATO has protocols for member nations to inform each other of the general area where each other's submarines will be operating. But France left NATO in 1966 (although it has been discussing rejoining), and does not participate in this submarine "deconfliction" program.
There's also the possibility that one, or both, boats were having problems with their passive sonar at the time of the collision. It's also possible that one of the boats did detect the other at a distance, decided to investigate further, and that all went badly.
The 428 foot long Le Triomphant class boats displace 12,600 tons, have a crew of 101 and carry 16 M51 ballistic missiles (weighing 56 tons each, carrying six warheads and with a range of 10,000 kilometers). The other three Triomphants, already in service, carry the older M45 missile (weighing 35 tons each, carrying six warheads and with a range of 6,000 kilometers). These boats will get the M51 after the "Le Terrible" enters service. The Triomphants replace the six SSBNs of the Redoutable class, 9,000 ton boats that entered service in 1971 and were retired in 1991. Each of these boats carried sixteen of the shorter range (5,000 kilometers) M4 missiles.
The Vanguard boats are a little larger (465 feet long, crew of 135), and entered service in the 1990s. They carry 16 Trident II missiles, weighing 59 tons, with a range of 11,300 kilometers and carrying up to eight warheads.
On February 6th, the French Navy announced that it always has two SSBNs available for duty, so that if one is unexpectedly put out of service, another is available to go out on patrol. Sea based, nuclear armed missiles are a deterrent to other nuclear nations only if you have one of your SSBNs at sea at all times.
Had these two boats hit each other sufficiently hard to cause a hull breach, and send one or both of them to the bottom, they would have joined many other nukes that have gone down since the 1960s. The nuclear reactors and warheads are built to stay submerged, and contain their radioactive material, for a long time. This has worked with the dozen or more other nukes that have gone to the bottom. Except, of course, for the Russian mass sinking of nuclear subs and reactors in the Arctic ocean in the 1980s and 90s. That was halted in the 1990s, because of the possibility of polluting Arctic fish stocks, by Western nations providing money to safely take apart and retire over a hundred obsolete Russian nuclear subs.