The missile that just popped out of the ocean, can hit any place on earth while potentially carrying a nuclear warhead … and it was launched from a submarine. This is what makes these underwater boats some of the deadliest pieces of equipment in any military arsenal … until you realize that submarines are really like giant cannons … made of glass, because they are only as deadly as they are stealthy. Once they are detected, the jig is up, and that underwater boat just became a giant underwater target.
This is why militaries around the world have poured billions of dollars into trying to keep their subs as sneaky as possible, while at the same time spending billions in figuring out how to spot the other guy’s submarines. But why being too stealthy can actually be a bad thing for submarines, how a small school of random fish swimming around is some of the best camouflage a giant sub can ask for, and why these underwater boats have to operate deeper when in the Arctic compared to the equator, is not what you think! The easiest way to pick up a submarine floating around underwater, is by its acoustic signature.
Every submarine has a unique set of sounds produced by its internal systems and engines, and even propeller movements. That’s why submarine propellers are typically covered when in drydock, to prevent enemies from deriving the acoustic signature based on the shape and size of the propeller. To minimize their acoustic signature, subs use active noise canceling mounts for onboard machinery, and specialized geometry on their hull to push through the water as smoothly as possible.
American Virginia-class submarines for example, on average produce around 90 to 95 decibels when cruising, which is roughly the same noise level as the currents in the ocean, and marine wildlife around them. This means that even a small school of fish looking for dinner, adds enough noise to the background to help submarines hide in plain sight. But that means that in areas where there is no wildlife to make noise, or the water is particularly calm, the crew has to ensure they stay extra quiet.
They call it “Silent Running”, because even having sailors walk around with boots on, or having a normal conversation could produce enough noise to be detected. Sea-wolf class attack submarines, which are the quietest submarines on earth, have an even smaller acoustic signature. Sounds great, but that might not always be a good thing.
One way that submarines are able to achieve such low signatures, is with anechoic tiles, which are these squared off pieces you see on submarine hulls. These are synthetic or rubber based panels that are installed in layers on the outside of the superstructure. They work in the same way that foam panels work in a recording booth, by absorbing soundwaves from the outside to prevent reflection from any active sonars.
But also from the inside, so that the noises inside the submarine don’t “leak” out. Because one of the biggest sources of noise on modern subs, is their powerplant. Modern nuclear power plants on submarines allow the boats to stay underwater indefinitely, as long as the crew doesn’t run out of food and toilet paper.
Ironically though, older diesel electric submarines are actually quieter. Diesel subs run off of battery power when underway, which means there are no moving parts needed to keep everything powered. But diesel submarines have the massive downside of needing to surface periodically, to charge the batteries with the diesel generators, which can take upwards of 8 hours in some boats.
This combined with the limited time the sub can operate before running out of fuel, means that most countries decide to just stick with the slightly louder nuclear option … if they can afford it! What’s surprising is that being too quiet can also be dangerous for a submarine. A submarine produces the same amount of noise, even when it’s operating in a particularly loud part of the ocean, with things like dense wildlife, or strong currents.
But if the anechoic tiles absorb most of the sound waves, they can create a quiet zone, known as an “acoustic hole”. This can be easily detected by Passive Sonar, as it comes across an unusually quiet space in the middle of all the ambient noise. Any well-trained sonar tech will be able to tell that there’s probably a sub floating around in the acoustic hole.
But guess what happens if the sonar tech wasn’t well trained! In the early morning hours of February 4th, 2009, the HMS Vanguard, armed with Trident II nuclear missiles, was on a routine patrol in the Eastern Atlantic Ocean. Everything was going normally until the sub collided with a massive underwater object that they had somehow completely missed on the sonar.
That object was none other than the French submarine, Le Triomphant, which was also armed with its own complement of nuclear missiles. Both submarines sustained enough damage to need emergency port repairs. It was later determined that despite both subs having the latest and greatest underwater sensors, neither was able to detect the other because they were both so quiet, which led to them unexpectedly bumping into each other in the ocean.
To prevent such accidents, navies may store the acoustic signatures of their submarines in a database. That way friendly ships and subs are able to look for those exact frequencies given off, without the submarines having to expose themselves to any nosey onlookers. But because those acoustic fingerprints are so useful in finding your own submarines, they’ve been hot tickets for spies to grab.
In 2018, Chinese cyber attacks managed to steal a mountain of classified data from the United States Navy. Experts believe that some of that data included the specific acoustic signatures of some American submarines, making it easier for them to find for China’s underwater detection networks. But if you knew there might be a submarine in a particular area, there is a way to find them from the sky!
Similar to their acoustic signature, submarines also have a thermal signature. This is the heat emitted from the hull, which can be detected by sensitive infrared sensors and thermal imaging systems installed on helicopters and satellites built specifically for anti-submarine warfare. Thermal signatures are a huge factor that determines how deep a submarine will be patrolling.
In areas closer to the equator where the water is warmer, subs can swim a bit higher if they need to. But in colder waters near the arctic, even the smallest heat signatures are easily detected, so subs have to stay lower. This is why submarines use the ocean’s thermoclines to their advantage.
Thermocline is the transition layer between the warmer mixed water at the surface and the cooler deep water below. If a submarine sits below the thermocline, any heat it generates would effectively dissipate before reaching the surface, so aerial thermal sensors wouldn’t be able to see them. The challenge is that thermoclines aren’t at a fixed depth and are constantly changing, so if submarines aren’t careful, they can accidentally reveal themselves to any kind of thermal sensors that might be looking for them.
The same aircraft looking for thermal signatures also could deploying these things, and no that’s not the crew’s trash from lunch. These are called sonobouys. Think of them like sonar bombs.
Once they hit the water, they deploy a hydrophone sensor, which picks up all the acoustic activity going on underneath it, and then transmits that information via a radio signal to the aircraft or ship that deployed it. You might think that an obvious thing to look for if you want to spot a submarine, would just be to look for any periscopes sticking out of the water. The thing is, submarine crews are very selective when and where they get close to the surface, and even more careful about sticking a giant periscope tube out.
Many countries will cover the periscope in radar absorbent materials, similar to a stealth fighter. That way no one would be able to pick it up with any kind of sensor. To get around this, the United States is looking for ways to detect the small waves that are produced by a periscope, rather than the periscope itself, meaning future subs will probably need a way to reduce the wake they produce when they do use it.
But what happens once a submarine is detected? One of the biggest threats to sub crews are torpedoes, which have a suite of highly advanced sensors for tracking. These are able to track every bit of positional data that an enemy submarine produces, from sonar and magnetic signature, to even the wake produced by the propellers.
A modern torpedo can even tell the difference between a real submarine, and a decoy designed to look like one. Then just like any other missile, once it gets close enough, it detonates. Keep in mind that being fast and sneaky means a submarine can’t get bogged down by a bunch of heavy armor, so while they look fairly sturdy, they can actually be pretty fragile.
Torpedoes these days are launched by pretty much anything above, on, or under the water. Anti-submarine helicopters come equipped with torpedo tubes as standard, many modern warships have some kind of torpedo launching capability, and of course, submarines have torpedo launch tubes that flood with seawater before firing. Old school depth charges used to be a favorite method of taking out a sub.
These would sink to a predetermined depth, and then automatically explode, either destroying the sub or forcing it to come to the surface. Some navies, like Russia’s, use something like the RBU-6000, which can spread a large number of depth charges around a large area. However, most modern submarines can now get around depth charges.
Arguably, the best way to catch a submarine by surprise is with a mine. You might be thinking of something that looks like this but these days you’re more likely to find something like the CAPTOR mine, which sits anchored on the sea floor, and fires a MK 46 torpedo at any submarines floating by that shouldn’t be there. The depth of the ocean may provide a lot of cover for submarines, but it also makes communications with the submarine almost impossible.
The high frequency signals that surface ships use for communications can transmit large amounts of data while being encrypted and resistant to jamming. But they cannot penetrate past the surface of the water. So how can submarines communicate with the outside world?
Submarines could use low frequency communication to get messages out, but these can be easily detected and tapped, so they only do it if they absolutely have to. The lowest end of radio comms, is the Extremely Low Frequency range, or ELF, which is able to easily penetrate deep into the water. It’s just that it carries so little information on it, that it can only be really used for one thing: launching nuclear weapons.
Nuclear submarines are part of the TACAMO program, or Take Charge and Move Out. In the event that the United States decides to start lobbing thermonuclear warheads around, specialized aircraft like the E-6 Mercury will fly over the patrol paths of American nuclear subs, transmitting launch data on the ELF spectrum. American submarines looking for these signals will typically deploy a long trailing wire, which needs to be large enough to pick up on such long wavelengths.
That said, those messages are one-way only, so once a boat receives the signal to start a launch sequence, that’s it, and they begin initiating a strike on their pre-designated targets. Luckily that’s a worst case scenario. These days, submarines do a lot more spying, wire cutting, and deterrence than they do blowing up aircraft carriers and launching nukes.
So more and more countries are investing into spotting them, like the Russian “Harmony” detection network, and China’s Underwater Great Wall, which looks for submarines in the South China Sea. That said, the submarine stealth technology continues to evolve, because in the world of submarines, it’s not ‘speak now or forever hold your peace’—it’s ‘stay quiet and forever hold your breath.
