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I am assuming that present day navies would hunt a sea monster using techniques similar to ASW (anti-submarine warfare). According to my research most ASW weapons like torpedoes, mines and missiles rely mostly on acoustic (sonar/radar) targeting, and possibly MAD (magnetic anomaly detection) or visual sighting.

Now, what if the sea monster in question has an ability to negate sonar detection via absorption of sonar waves (for a short period of time) and jamming by flooding the sonar scopes with false positive readings (deploying swarms of little copies of itself), thereby making real detection only possible via a visual aspect—a very dangerous way of fighting it. MAD would also be useless since it has no metal bodyparts.

What kind of tech/strategy would navies use to combat such a creature?

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    $\begingroup$ Techniques for tracking a sonar absorbent object will differ greatly from those for tracking a source of jamming signals. So which is it? Or is it capable of both? $\endgroup$ – Henry Taylor Apr 28 at 16:36
  • $\begingroup$ I'm thinking it can do both. Absorb sonar for a very limited time. $\endgroup$ – JTriptych Apr 28 at 16:50
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    $\begingroup$ Isn't there a way to track abnormal water movement, to be able to detect stealth ship ? $\endgroup$ – MakorDal Apr 29 at 5:52
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    $\begingroup$ I think you're getting a lot of answers that suggest how to detect a 'creature that absorbs sonar' - which is giving you answers you might not be looking for. A better idea might be a creature with a body that absorbs no sonar - not reflecting it, not absorbing it, but passing straight through it with no effect at all. $\endgroup$ – Zibbobz Apr 29 at 14:07
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    $\begingroup$ Right, absorption is the wrong way to go. That would be like painting an object with Vantablack, and expecting it to turn invisible because it's absorbing all the light the strikes it - instead, you get a very conspicuous silhouette. $\endgroup$ – Nuclear Wang Apr 29 at 14:35

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Frame Challenge from a Submariner

It doesn't need to have magic anti-sonar powers.

Three reasons:

  1. Sonar is pretty limited in range. The ocean is large,[citation needed] and each platform can only usefully employ sonar in a limited area. Think tens of miles for very noisy targets, and on the order of a mile for quiet targets.
  2. Sea Monsters aren't ships - Passive ASW tools are designed to find the rotating machinery on other ships (screws, engines, pumps, fans, whatever). A sea monster isn't going to have those parts.
  3. Sea Monsters still aren't ships - Active ASW is designed to bounce off the metal hulls of ships... which sea monsters don't have. A rounded, blubbery hide is going to absorb / scatter active sonar fairly well.

So existing tools are going to struggle to find your sea monster. Assuming that the monster can navigate the ocean depths without using its own active sonar, it's going to be very quiet - like don't 'see' it until its three hundred yards away and closing quiet.

And if you use active to try to find it, you may extend your detection range to a few thousand yards, but you've also told the monster where you are....

This is, in fact, fairly similar to hunting submarines. The quietest submarines out there are very, very, very hard to locate. A whale (or sea monster) that's not singing or echo-locating is probably about as quiet as the quietest submarine.

Which is pretty scary.

And as for Actually Hunting it

Transients. Unusual noises associated with its operation.

What does it sound like when the monster kills and eats some other sea critter. (it's got to eat, right?)

Does it come to the surface to breath like a whale?

Does it use its own sonar to navigate?

Does it talk to other sea monsters to coordinate attacks or mate?

All of these could be fairly loud, and could help localize the monster.

Once you've got a rough idea of where it is, you vector in other assets (read submarines) and they do the whole "cat and mouse" thing. Active sonar only comes into play in the final seconds, if at all.

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    $\begingroup$ Thanks, you and the others have given me a lot of ideas. Now to finish the book! $\endgroup$ – JTriptych Apr 30 at 6:27
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    $\begingroup$ Also maybe the monster does mating calls. $\endgroup$ – Steve Bennett Apr 30 at 7:12
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    $\begingroup$ If I have a "rough idea of where it is", namely in Loch Ness, this would be feasible? $\endgroup$ – Hagen von Eitzen May 1 at 10:07
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    $\begingroup$ To elaborate on transients: IIRC at one point submarines got too quiet, so one technique for trying to find one was to look for a "hole in the water". A large solid body blocks sounds generated behind it, so essentially you look for where the transients aren't. $\endgroup$ – Craig S. Cottingham May 1 at 15:18
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It would be a lot like fighting submarines in WW1 and WW2. Sonar wasn't really that good; destroyers usually would only notice a submarine after it torpedoed something they were protecting. Sonar would be used to pinpoint the enemy's position, not detect it in the first place.

So ships would start sailing in convoys with armed escorts. Presumably this sea monster attacks with some kind of melee weapon, meaning it must be at or near the surface and adjacent to its victim; completely vulnerable to naval gunfire. If it survives that, the escorts can drag big, barbed nets around. Modern destroyers are pretty fast, the Arleigh Burke class can go at least 35mph, probably faster too as its true top speed is classified. Can a sea monster do 35mph or more for hours on end without tiring? Because it'll have to if it wants to escape a squadron of Arleigh Burkes.

If the sea monster has a ranged attack, it's trickier but the nets could still work. Submarines in WW2 were often sunk and they could launch torpedoes from a few kilometers away, so distance is not a panacea.

Worst comes to worst, booby trap some tasty-looking ships. It bites the wrong one, and boom, the nuclear bomb inside goes off. Lets see anything biological survive being adjacent to a W88 when it detonates.

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    $\begingroup$ I'm enjoying the mental picture of the noble sailors with the baited ship knowing they will soon die from the explosion, but hopefully the evil creature will perish, so their sacrifice will not be in vain. (Assuming this period is before remote controls or autonomous AI operations.) $\endgroup$ – Paul Williams Apr 29 at 20:02
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    $\begingroup$ @PaulWilliams The ship only needs to be crewed near shore. You could easily have the crew get it into open water, set a straight course, then abandon ship. If its course needs changed, a new crew can board it long enough to make that happen. $\endgroup$ – Ryan_L Apr 30 at 2:59
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    $\begingroup$ Have the crew in a balloon on a tether. In case of monster, cut the tether and float away. Also, the monster is biological in nature. No need for atom bombs. There are plenty of chemical nasties. A ton of sodium, carefully stored in water-free oil will do nicely. When its container breaks somewhere in the intestines of the monster, the resulting reaction will very violently turn a significant part of the monster's organs into hot hydrogen gas. No need for cleanup, the sharks will find the remains. $\endgroup$ – MSalters Apr 30 at 10:57
  • $\begingroup$ Because it'll have to if it wants to escape a squadron of Arleigh Burkes It could also dive $\endgroup$ – WoJ Apr 30 at 14:10
  • $\begingroup$ @WoJ Nets can be pretty deep. $\endgroup$ – Ryan_L May 1 at 4:17
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A perfectly absorbing object to a sonar beam would look like a perfectly black object to visual observation: it would still stand out on the background because it would cover it, especially when observed from above.

It's just the tracking software that needs to be properly configured.

And don't forget that underwater explosions deal damage even at a distance, thanks to the properties of liquids. Therefore once a ship is attacked it can simply deploy depth bombs and at least scare away the monster.

If the ship is not alone it's possible to use group strategies, too.

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It sounds exactly like a modern submarine, which is also designed to absorb or deflect sonar, can launch decoys and noisemakers and otherwise use (mainly classified) gear to prevent itself being spotted.

So, ASW as usual.

However, if I was being creative I'd use the US Navy's Marine Mammal Program which uses dolphins and sea lions for anti-mine warfare (and, allegedly, other purposes) or its Russian equivalent, and do some quick retraining to teach them to track down sea monsters.enter image description here

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    $\begingroup$ Plus you know the Russians extended their program to include other … let us say "less common" creatures. $\endgroup$ – Willk Apr 28 at 21:53
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    $\begingroup$ @Willk Something Something mind-controlled giant squid... $\endgroup$ – Ruadhan Apr 29 at 8:14
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    $\begingroup$ @Willk - Pretty sure we now know where OP's monster came from to begin with ;-) $\endgroup$ – Kevin Apr 29 at 14:02
  • $\begingroup$ In the context, I want to read "other purposes" as "other porpoises". My brain just does these things to me. $\endgroup$ – Darrel Hoffman Apr 29 at 16:22
  • $\begingroup$ @Darrel Hoffman Yes, I wanted to write that :) $\endgroup$ – David Hambling Apr 29 at 19:29
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Don't shoot a warhead at it. Trick it into eating the warhead

This thing is a sea monster. What does it want? Why is it attacking ships? Either because it sees ships as prey, or as competition for its prey, or as threats to itself or its kind. If our navy is hunting it at all, it's because it's attacking our shipping.

So unless it's shooting Godzilla breath or some other kind of death from afar, some part of this creature is coming into close contact with ships. Figure out what's likely to provoke an attack, load a ship to its maximum weight full of explosives and bait it into attacking.

This might not kill it (depending on how big it is, or if it's using tentacles from far away, or something) but it will definitely not like that much very much. If you get lucky, you kill it. If not, it will get a lot more reluctant to attack ships.

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    $\begingroup$ No, it's not attacking anything. The navy is hunting it because they want to study its superb sonar invisibility! $\endgroup$ – Bergi Apr 29 at 17:01
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  • (Active) Sonar has nothing to do with radar. It emits sound waves which get reflected by the target. If the target absorbs the sound waves, there would be a detectable "hole in the ocean."
  • (Passive) Hydrophones listen to the sound made by the target. The monster flips a fin, water flows, and there is sound. The monster filters water through gills, water flows, and there is sound.
  • (Active) Ladar or Lidar emits light waves which get reflected by the target. They're considered for various uses which involve spectroscopy, while ASW uses are more experimental.
    One potential benefit of ladar are very exact distance measurements, which can detect the "bow wave" of the monster from above the sea as well as the monster directly.
  • Those waves also cause small sea animals to glow, which can be detected.

So: The monster isn't as stealthy as you think it is, unless you give it more fantastic capabilities.

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    $\begingroup$ Small correction: Lidar/Ladar uses light (that's the "L" in the name) - laser to be exact - , not sound. $\endgroup$ – Syndic Apr 29 at 5:19
  • $\begingroup$ @Syndic, yes, I'll fix that. $\endgroup$ – o.m. Apr 29 at 14:27
  • $\begingroup$ Lidar could work if you know what ranges of the spectrum the creatures are able to detect. It's not the same for all creatures. Some animals can see more frequencies than humans, into the IR and/or UV parts of the spectrum. Some can see less - e.g. dim red light is often used by nature documentarians to observe nocturnal animals since many animals cannot see it. Sensors can be calibrated to any frequency, so you could pick one beyond the monster's visual range if you could determine what that is. $\endgroup$ – Darrel Hoffman Apr 29 at 16:32
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According to my research most ASW weapons like torpedoes, mines and missiles rely mostly on acoustic (sonar/radar) targeting, and possibly MAD (magnetic anomaly detection) or visual sighting.

Well, radar is not acoustic, it's electromagnetic. But in any case, as you point out, there are many different detection mechanisms and so they would try them all until one of them happens to work.

I suppose you could declare that the monster is invisible to every single detection method. If it is at least visible, and goes to shallow water, you could find it with spotter planes. Then simply drop depth charges in the area and see if a bunch red stuff floats up... Assuming the blood and guts aren't also invisible.

If it is even invisible to sight, okay, no big deal. Even if it's acoustically invisible, things that move fast in water lead to cavitation which is sound generated by water itself. Not much the monster can do to hide that, unless it has a way of moving fast without cavitating. If it does, no sane navy would destroy it under any circumstances - it would be worth its weight in gold ;-).

The one possible way around cavitation is decoys as you mention... But then this is just the one monster, and kind of an important target. Can't they just shoot torpedoes or depth charges at every decoy? How many decoys does it make? At some point you can triangulate the monster's true position simply from statistical analysis of the decoy locations.

In any case, locating the monster is super easy. You just tag it with some kind of transmitter. When it's detected in an area, drop a bunch steaks with an active transponder (sound, radio, anything) inside, so the monster swallows them. Or if it only likes boats, send some unmanned drone ships with the same. If it doesn't like the drones, well, no pain no gain - just mandate transponders on all civilian ships (or maybe you can simply home your torpedos/missiles/whatever on the existing transmitters the civvie boats have) so that no matter what it eats, it will swallow one.

Granted, if you can make it swallow things, why not just feed it a bomb? I suppose you want to capture it intact, for the stealth sub technology... In any case, if feeding it transmitters doesn't work for some reason (magically digests everything?) then you can station riflemen on every ship or convoy that shoot a transmitting dart when they spot the monster. This would work even if it's invisible - you'll see the ship being eaten, so just shoot at the air around it.

Depending on the size, speed and habits of the monster you could deploy large nets. These nets could have some kind of transmitter which would once again enable detection. Not very eco-friendly -- but it sounds like there's only the one monster, and a comparatively small piece of ocean you would disturb.

Lastly, there is always the overwhelming firepower route. You could have patrols by planes, helicopters and fast ships in the area it is spotted, even escort civvie convoys with warships. As soon as the monster attacks, immediately fire cannons, cruise missiles and the like. If it's moving around too much, you can just target the ship being eaten, depending on how long that takes. You can also just launch several speculative ones in directions it's likely to go next. Since there's only the one monster, being efficient with munitions is presumably not a concern. And of course, there's always the literal nuclear option. Again, not very nice to the fishies, but they used to do it all the time for testing, and it's really just the one time. What's one more underwater nuke?

But the more I think about it, the more I wonder, why would the military want to destroy it? If it's giving them so much trouble, then it's extremely valuable for the development of submarine technology. I imagine they would try to capture it alive by luring it into some cove, or at least kill it without destroying the carcass. While this delicate process is underway, they could deploy many unmanned decoy ships to minimize the danger to civilians.

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Harpoon it with a locator beacon.

There could be several kinds of locator beacon. A simple one just emits a sonar ping every so often, which can be picked up with a passive sonar detector (and would thus be immune to the monster's sonar absorbtion). Another kind would be like an EPIRB emergency locator beacon. They only work on the surface, but whenever the monster is on the surface, it can be tracked worldwide.

A combination would make the monster fairly easy to track.

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Quantitative DNA analysis of seawater.

A Splash of River Water Now Reveals the DNA of All Its Creatures

Environmental DNA, or eDNA, is at the center of a brand new kind of fish and wildlife biology, and it is such a powerful tool that it’s transforming the field. eDNA was first used to detect invasive bullfrogs in France a decade ago. It was used in North America for the first time in 2009 and 2010 to detect invasive Asian carp in and around the Great Lakes. Since then, its use has grown exponentially, primarily in marine and freshwater environments.

“You can’t manage a species if you don’t know where it is — even 80-pound Asian carp, because you can’t see them underwater,” said Cornell University biologist David Lodge, who participated in the Asian carp study. “So eDNA is particularly powerful in aquatic systems.”

Things shed DNA into the environment. You can tell what is in an aquatic environment by quantitatively analyzing the water for DNA. If this can be done quickly, you could lay out the large area your creature might be and sample water around the perimeter, then narrow your sample area down. When you get a hit you assume the hit is from the outside edge of where the creature has been lately and quickly bring your sample circle down in size. As you sample closer to where the creature has been lately, the concentration of its DNA in the water will increase.

Once you have the area narrowed down to a manageable area of several square kilometers, comb through that area visually using submarine drones with water-penetrating imaging lasers.

drone

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Just a comment or three.

If you ping an area of ocean, you get an echo for any object in that direction. If there is a sea monster there, you expect (or hope for) an echo. If you do not get one (because the monster does not reflect)… that is not analogous to a big black thing; it is the same as the [other] empty areas. The only case in which you would “see a hole” is if you are near a large “object” such as the ocean floor. (In that case, the monster would have to return the “ping!” when it felt the rest of the return signal. Like a lyrebird. A big, grey, bad-tempered lyrebird with huuge fangs… .)

In the case of human submarine vs ditto… one must oneself remain quiet. Depending on the details, that might well not apply against a sea monster. If it is hunting you, you need to try to be invisible in whatever way is applicable… or be more dangerous. On the plus side, it makes it easier to find. If it is not hunting you, then you can use whatever you can — really loud sonar, lasers, probes, trained dolphins, nuclear weapons, fields of sunken sonar buoys, food, lights, methane traps, biological warefare, herding… .

I am guessing that the scenario that the OP has in mind is hunting an animal that is minding its own business. In that case, the biggest problem (nod to codeMonkey) is how big the ocean is… or, putting it another way… how hard it is to see any useful distance underwater. You would be well advised to start with studying its behaviour. You might also like to work on seeing underwater, but that is why they invented sonar in the first place.

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There's an entire host of sensor technologies out there that can detect things in water. Sonar works by sending sound waves thru water to impact and resonate back to whatever it interacts with. So unless a creature is "Sound Proof" that's unlikely. Sonar doesn't look for the object it listens to the sound made as it travels. Passive sonar listens to sounds made by the object moving thru water, Active sonar emits it's own sounds and hopes for a feedback.

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Track its path by looking for abnormal genetic material or biochemical markers that don't match any known sea creature in the area. Once you find something abnormal, it's safe to assume that it's a sea monster.

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