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In my fictional world, there is a race of mermaid-like creatures that are having a bit of a problem. That problem is that the surface dwellers have made something called a depth charge. This makes underwater explosion that creates pressure waves that crush their lungs. So what I’m wondering, is if there is any way (aside from swimming) to protect against underwater pressure waves?

This race has advanced biological technology from breeding and growing their tools. It is at the point where they have taken a dolphin and breed everything except the sound organ to non-existence and turned the rest into a sound gun.

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    $\begingroup$ oh wow your surface dwellers are disgustingly brutal... $\endgroup$ – theonlygusti Oct 28 '19 at 10:25
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    $\begingroup$ Why do the mermaid-like creatures have lungs in the first place? Having lungs implies that they must surface for air. Any fish worth its weight is going to have gills. No lungs to collapse, no problem! $\endgroup$ – MonkeyZeus Oct 28 '19 at 19:00
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    $\begingroup$ Because mammals have lungs. $\endgroup$ – Seraphim Oct 28 '19 at 19:46
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    $\begingroup$ Would a metal breastplate be feasible? The depth charge would probably pose concussion risks too so football helmet? $\endgroup$ – MonkeyZeus Oct 28 '19 at 19:58
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    $\begingroup$ @MonkeyZeus I see you're getting your info from SharkSider. But most references I've found frame aquatic diets in terms of percent body mass. These mermaids are aquatic mammals, and mammals are costly, so I think a dolphin is a better comparison. TheSuperFins, SharksWorld and SharkProject say a shark eats at most 3% of its body mass every day. DolphinCommunicationProject, UCSB and WhaleFacts say a dolphin eats 4-9% of its body mass every day. "A great white [...] only uses 0.2 calories of energy per kilogram of body weight every hour. Humans use eight times as much energy", TSF notes. $\endgroup$ – undine_centimeter Oct 29 '19 at 15:19
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Bubble curtain.

bubble curtain around oil rig https://www.eurotrib.com/comments/2012/9/6/165418/5127/3

Depicted - an oil rig with a circumferential bubble curtain to reduce underwater noise.

https://en.wikipedia.org/wiki/Bubble_curtain

A bubble curtain is a system that produces bubbles in a deliberate arrangement in water. It is also called pneumatic barrier. The technique is based on bubbles of air (gas) being let out under the water surface, commonly on the bottom. When the bubbles rise they act as a barrier, a curtain, breaking the propagation of waves or the spreading of particles and other contaminants...

It can be used for the following purposes: to reduce propagation of shock waves (e.g. acoustic waves from engines or pile driving,1 explosions etcetera)...

Your mermaids need some tech to produce gas. If they can breed dolphins into handheld squeak guns, making bubbles should be within their abilities. You could ferment things, or tap deep methane deposits, or any number of things. The curtain might be constant or could be turned on when the mermaids hear boats coming.

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    $\begingroup$ This could interfere with underwater communication. $\endgroup$ – Paul Oct 28 '19 at 8:17
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    $\begingroup$ @Paul: Well, armor (plate armour, tanks, bunkers …) interfere with sight and sound as well but we’ve found ways around that. $\endgroup$ – Michael Oct 28 '19 at 8:59
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    $\begingroup$ Seems like this wall would only help if the depth charges fall outside it while mermaids are inside it (or vice versa). How should the surface dwellers be prevented from dropping depth charges inside the wall? Maybe this is where the kraken comes in. :-) $\endgroup$ – LarsH Oct 28 '19 at 14:47
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    $\begingroup$ @LarsH you could have an air filled cavern separating each of their buildings from the rest of their underwater domain, that way as long as they're inside of their buildings they're safe from pressure waves $\endgroup$ – BKlassen Oct 28 '19 at 15:48
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    $\begingroup$ @LarsH - it is sort of like a trench during an artillery bombardment. Down in the trench you are protected from shells landing nearby, but not from a shell that enters the trench and lands in your lap. $\endgroup$ – Willk Oct 28 '19 at 15:56
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A bubble curtain is a decent idea, but for something more static, that doesn't require continuous power...

A sponge wall.

You want a series of baffles that will allow water to pass through, but will choke and disorganize the flow, absorbing and dissipating shockwaves. Sponges will do that just as well in water as spongy materials do in air to absorb sound.

If you want something that a person-sized creature can swim / crawl through to enter and exit and enclosure, you build the same kind of structure, just at a large scale, with person-sized twisty passages rather than food-particle-sized ones. Those could be larger passages carved through a thick wall of bioengineered sponge, or you could use other, stronger materials--e.g., a custom grown rocky coral reef.

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    $\begingroup$ you could also engineer sponge walls with gas pockets inside (produced by the sponge itself maybe, or by some other micro-organism). gas would help a lot absorbing shockwaves. $\endgroup$ – carlo Oct 28 '19 at 15:47
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Kraken

Can't drop depth charges if your ships have been eaten.

enter image description here

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    $\begingroup$ Pressure-resistant Kraken! $\endgroup$ – DVNO Oct 28 '19 at 14:29
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    $\begingroup$ Kraken not bothered by pressure waves. Kraken eat depth charges for fun. Kraken like tickly feeling. $\endgroup$ – Whelkaholism Oct 28 '19 at 15:28
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    $\begingroup$ Won't solve. Torpedoes, sonars, bigger ships (Yamato-size) and nuclear depth charges will kill any kraken the merfolk can pull $\endgroup$ – Geronimo Oct 28 '19 at 17:03
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    $\begingroup$ Also, I think planes can fire depth charges as well. If not, then there are still large weapons that they can. $\endgroup$ – Mark Gardner Oct 28 '19 at 20:29
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    $\begingroup$ Big kraken have Stalium stomach! Big kraken likes radioactively In lunch! But seriously, using a nuke as a depth charge would get the job done, I wouldn’t want a riot of every coastal city storming me. $\endgroup$ – Seraphim Oct 29 '19 at 3:03
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By stopping the waves at the source

These surface dwellers don't seem to know or care about this race. Of course, you can put up a shield, or breed sea monsters to deter approach, but that might actually make the land-folk more curious, and encourage them to come closer. When they find Atlantis the thing being defended, all the while being encouraged by continued losses, the will find Aliens! a hostile, hidden force. I predict very fast escalation, if not straight-away nuclear strike. Not an optimal outcome. This is, of course a simplified timeline, but unless peace is reached very fast, a war of extinction is quite probable.

What would the average country with a sizable navy say, if they found that the Bermuda triangle is full of dinosaurs things who have secretly developed genetic hacking to such an extent that they can breed dogs into guns? They would quite probably assume that targeted diseases, i.e. biological warfare, is only a short design process away, or even stockpiled. Some trigger-happy general or president might decide to keep this tech out of any potential terrorists or enemy countries, by destroying it.

I think that I've made it pretty clear that hiding is not a very viable option. Instead, I recommend one of two options:

The best defense is an indiscriminate offense

Since you can indeed breed things to such high usability, you could easily breed a perfect biological weapon (Just a few options). Or, you could go for a less overt option, and let them wipe themselves out. While hijacking a sub, and getting it to launch a nuclear missile would be quite difficult, there are other, similar options. If your ethics committee is OK with it, you could breed one of your own to be able to pass as human, and become a political leader who can start the war themselves.

If nuclear fallout is not a desired side affect, and neither is extinction, or even a race of slaves, you can always try

The best policy, honesty.

If it wasn't apparen't already, I'm assuming that the surface-dwellers have no idea you exist. If they do, then they have basically been dropping bombs on you. They asked for what's coming. However, you could try giving a few examples of why they don't want to do that. This might make you a few enemies, but if you make the demo impressive enough, they won't dare attack you. Or they might try their best to wipe you off the planet, but to be fair, they were already trying to do that anyway.

If they don't then the remedy is simple: Say hello. (This one has a more impressive "greeting" on the poster). How you go about doing this is up to you. You could breed a Kraken, and show up riding it into New York harbor. You could send Morse code through your specially bred whale orchestra. You could even go through Area 51. However you do it, eventually you will be able to go up to the UN, and demand that depth charges be banned.

For a more impressive, if more risky approach, secretly sponsor a splinter group to start along the violent route. Then urgently contact a group of politicians, warning them of the threat, gaining yourself some friends, but also sending the message that these things need to be stopped.

This also has the bonus of letting the community make space whales within the foreseeable future.

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    $\begingroup$ This a really good look into the political aspect I never thought of. The only issue is that the two races have known each, for really long time. Like prehistoric long time. Now, during all these centuries, the main conflict between the two were territorial disputes and that fact that the damn land dwellers keep stealing our food! Now, the majority of groups have coexisted peacefully but some are constantly at each other’s throats. But due to the environment, the land dwellers always lose. Until depth charges were invented. At least, that’s what the inventor hoped. $\endgroup$ – Seraphim Oct 28 '19 at 22:47
  • $\begingroup$ @Seraphim so, the all-out war option it is. Depth charges can be almost as destructive as nukes, possibly more so. You have the upper hand. Use it. $\endgroup$ – Mark Gardner Oct 29 '19 at 17:56
  • $\begingroup$ definitely. This situation is why I posted this question in the first place. I first tried to search on my own but only got websites talking about warship armour, not that it wasn't useful, it just wasn't what I was looking for. $\endgroup$ – Seraphim Oct 29 '19 at 18:06
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Yell at it

The mer-folk could have their cetacean "sonic guns" create a countervailing shock wave. Since the energy of the depth charge is shared over the surface area of an expanding sphere, a very loud directed sonar blast by a dolphin / whale might disrupt it over a small section.

Blue whales can produce 188dB and sperm whales can supposedly generate a 230dB pulse (https://roaring.earth/sperm-whales-can-vibrate-humans-to-death/). Add your many generations of breeding and you might be able to mitigate a depth charge pressure wave.

The idea is a little similar to explosive reactive armour.

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    $\begingroup$ Cute, but ultimately futile. Two colliding waves will pass through each other with minimal effect. You need to disrupt the energy passing through the water, which an 'opposing' wave won't do. At best if you got the frequency and amplitude just right you could create a line of cancellation with interference patterns outside that line being potentially more damaging than if you do nothing. $\endgroup$ – Corey Oct 30 '19 at 6:09
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If they don't breath air and perhaps travel along the sea floor, the mermen wouldn't have an air void. If there is no air void in the creature, the pressure wave wouldn't really do anything to the creature.

Water is an incompressible fluid. Other than a 'supersonic wave' or cavitation, no damage would happen.

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    $\begingroup$ The question specifies that the creatures have lungs, which means they breathe air. $\endgroup$ – overlord Oct 28 '19 at 21:13

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