A couple of years ago, two ballistic missile submarines, one French, one British, collided underwater without being aware of each other's presence. The French initially ascribed the damage to "an immersed object (probably a container)". Only after Britain informed France that one of her submarines had also suffered damage did the two nations put two and two together.
The incident illustrated one of the conundrums of modern stealth technology (which is used on ships as well as aircraft). If you make a ship, or a submarine, stealthy enough, it's impossible to see or hear it coming in time to avoid a collision. In the case of ballistic missile submarines, whose patrol routes and routines are amongst the most closely guarded military secrets, there's not going to be a Notice to Mariners warning of their presence, so other traffic will remain oblivious to them unless and until they run into each other (literally).
It looks like France is trying to develop technology that will overcome this problem - although how successful they'll be remains to be seen. Strategy Page reports:
In early 2021 France officially began development of its next generation of SSBNs (ballistic missile nuclear powered submarines) ... These new SSBNs will be about ten meters longer than the current 138-meter long SSBNs and probably displace (on the surface) at least 15,000 tons. This is about twice the displacement of the first generation SSBNs. The additional internal volume will be largely devoted to silencing, improved passive sensors, much upgraded electronics and better crew accommodations. This includes a towed sonar array in addition to built-in sound detectors. More powerful computers and software can more quickly identify known sounds and figure out what unknown sounds are likely to be and track them even if the sound signal fades in and out.
. . .
It was confirmed that the hull of the 3G SSBNs will be covered with sonar signal absorbing anechoic tiles. Inside the subs all machinery will be designed and mounted to minimize noise ... For an SSBN the best protection is silence and stealth. All this stealth has a downside. This was demonstrated in early 2009 when France revealed that the Le Triomphant had collided with some unknown underwater object, causing some damage to its sonar dome ... Ten days later, after the British announced that one of their SSBNs had also collided with something underwater, they compared notes with the French and concluded that the two boats had run into each other ... The French revealed that onboard the Le Triomphant the collision could be heard by the crew as bumping and scraping, then silence. There was a damage assessment drill, which came up clean, except for some damage to the sonar dome. That 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 Vanguard did the same, unaware that they had literally bumped into another stealthy SSBN.
. . .
The odds of an underwater collisions were considered so high as to be virtually, but not absolutely, impossible. The U.S., Britain and France quietly got together to work out new procedures to avoid collisions between their increasingly stealthy subs. This is one reason why the French 3G SSBNs are spending a lot of money on improving passive sensors that will detect stealthy subs that get within collision range.
There's more at the link.
I was puzzled to read that. If every effort is being made to reduce the noise made by a submarine, how will better sensors be able to hear its reduced noise? I called an old friend, a man with whom I worked on some defense projects back in the 1980's. He'd been involved with developing new and upgraded sensors for South Africa's 1950's-vintage Mark 44 anti-submarine homing torpedoes, and understood the problems involved in detecting submarines. What's more, his son is currently working for a European military vendor that's trying to solve the same problem, only in a far more modern and technologically sophisticated way than he or I could have imagined, back in our day.
Jacques was pleased to hear from me, and spent some time describing the problem. Paraphrased, it went something like this: "You know how, when a sound source passes behind something that blocks it, the sound you hear changes until it's gone past the obstruction? After that, it goes back to normal. Well, that's what they're trying to do with a stealthy submarine. They may not be able to hear the submarine itself, but if sounds made beyond it are blocked by its passage, those sounds will change, even if only imperceptibly. If you can pick up that change, you know there's something blocking the sound - and if you're underwater, with no natural obstacles anywhere nearby, that could be a submarine."
I knew what Jacques meant. We live in a town through which passes a busy railway line. Sometimes trains are parked on a parallel track to clear the main line for another, more urgent train. When the latter passes behind the former, its sound is blocked by each successive rail car, so that it's a loud-soft-loud-soft progression of noise. It can sound very strange when the driver is sounding his horn, because that sound, too, ebbs and flows. The stationary rail cars act as obstacles to the sounds from the traffic passing behind them.
Still, that sound effect underwater has to be much, much harder to detect, particularly over ranges of miles, or tens of miles. It must be a heck of a technological challenge to identify it. Jacques agreed. He told me, "We're looking for a hole in the water - or, at least, a hole in the natural sounds in the water. If we find one, there may be a submarine in it."