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In the story I'm writing, there is a global civilization of marine life that's parallel to human civilization on land, however in its world, humans have little-to-no knowledge of the underwater civilization, while the underwater civilization knows about humans and actively tries to stay out of detection from humans.

I would like to know how the civilization of marine life would be able to avoid human detection, and ( if this isn't off-topic) to defend themselves in the case that a human did detect them so that a human doesn't get to escape to tell the tale.

These are the base conditions of this world:

  • As this underwater civilization is a parallel to our human civilization, and this story's timeline takes place between 2017-2020 the underwater civilization would have access to similar modern technology that we would currently have access to in the real world, except a lot more of it would be water-proof of course.

  • Like human civilization, this underwater civilization spans world-wide in the form of various nations, and territories (rather than the whole civilization being one collective entity in one set location).

  • The geography of this world is a nearly 1-1 to Earth, with the exception of a few fictional fully submerged islands/landmasses where open-ocean would've been in the real world, where some nations are lucky enough to call home (however most others do live within real world geography).

  • Humans in this world have similar knowledge and access to locations in the ocean as they do in the real world; ship trade routes, marine wildlife reservations, fishery locations, oil mining locations, scientific research locations, etc. so the underwater civilization is aware of these areas and would either avoid those places, or would find ways to limit or hide their activities there.

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    $\begingroup$ You are going to have to give some thought as to how such a civilization developed without fire. Either it is radically different from the terrestrial civilization, or it's a very recent off-shoot that deliberately hid itself in the ocean. $\endgroup$
    – chepner
    Feb 16 at 15:06
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    $\begingroup$ Go deep. This is basically the premise of "The Abyss." We know very little about what goes on in the deepest parts of the ocean. $\endgroup$
    – stix
    Feb 16 at 20:08
  • $\begingroup$ Question: Are your underwater sea dwellers mammals, reptiles, fishes, or invertebrates? This is going to strongly affect where your species can live. Mammals and reptiles can't live too deep because they need to breathe air. Many fishes are also unable to inhabit the deepest layers of the ocean because of their internal skeleton and presence of a swim bladder, which is liable to rupture under great pressure. Cephalopods mostly rule the deepest seas. $\endgroup$ Feb 16 at 22:09
  • $\begingroup$ @chepner I was thinking along the lines that the underwater civilization utilizes magnesium fires, since those types of fires can survive underwater, and magnesium is an abundant mineral in the ocean. $\endgroup$ Feb 17 at 19:10
  • $\begingroup$ @user235714 The overall civilization is made up of multiple groups of species; mostly bony fish, cartilage fish, and cephalopods, though a smaller portion of the overall civilization are indeed marine mammals and reptiles as well. $\endgroup$ Feb 17 at 19:21
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It would depend heavily on their tech level. It would be relatively easy for a Stone Age hunter-gatherer or an iron-smelting civilization to hide, but it would be a lot harder for any civilization that has become industrialized. One of the characteristics of high-tech human society is a huge ecological footprint: large areas cleared for farmland, megafauna wiped out either because they pose a threat or are an easy source of food, that sort of thing. Perhaps most notably, humans would keep noticing non-human artificial objects washing up on shore similar to giant squid, and if they are industrialized they would notice that someone seems to be contributing to global warming that isn't human.

That said, it would actually be a lot harder to hide a civilization in the sea than science fiction and nature documentaries make it out to be. Yes, the sea is unimaginably big, we haven't explored most of it, and there are lots of deep sea crevasses where a species could hide. The problem is most of the ocean is relatively empty.

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Most of what we think of as "the ocean" is actually what is often described as a "marine desert". There really isn't a lot there. The biomass here is incredibly low per square km, and most of the non-planktonic wildlife are larger animals that can migrate long distances in search of food (e.g., oceanic whitetip reef sharks, many other large sharks, tuna, marlin, manta rays, baleen whales, dolphins, etc.). Food is very scarce and animals will often travel for miles to congregate at food-rich locations. But more importantly there is almost no standing biomass beyond phytoplankton, because there is no bottom, and hence no algae growing on the seafloor. Light diffuses very quickly and photosynthesis is possible over a relatively short depth.

The deep sea floor (bathypelagic onwards) has a lot of space to hide, but nutrients are incredibly scarce down there. Many species in this zone can go years without eating and have incredibly slow metabolisms to survive the long periods between food. Food itself is mostly limited to whale falls, marine snow, and what else these animals can catch.

The mesopelagic zone actually contains about 95% of Earths marine fish biomass, but it's not clear if that's due to the fact that the zone is just so big or that humans have just depleted coastal fisheries so heavily and only the really inaccessible fishes are left. Food is still said to be patchy here. Also there is still the problem of no bottom, and thus no easily accessible mineral or stone deposits to build tools, space to grow crops, or places that can support buildings.

The problem is the good regions to build an aquatic civilization are all on the continental shelf, the region that extends about 65 km (40 miles) off the shoreline and is only about 60 m (200 feet) deep. That's actually the sunken edges of the continents, and beyond that the seafloor plunges straight down pretty sharply until it hits the marine crust (which is often a couple miles/kilometers deep). This is where most of the marine life people know of lives, and it's the region that has the highest primary productivity. It has the most habitat diversity and the most species diversity, being the zones that coral reefs live in. It's also the part of the ocean in which there is a well-defined sea bottom that still has light reach it, and therefore abundant algae, space to build structures, as well as access to stones and other natural materials like iron ore, corals, etc. All the sediment coming off the land also helps with fertility. The continental shelf is the ideal area to build a civilization.

Problem is, this is also the region of the ocean that is most easily accessible to humanity, and the one which humans have been fishing for thousands of years. Humanity's increasing pressure on the coastal ecosystems would eventually clash with the aquatic civilization, and the two would be forced to compete for resources. Given how many continental fisheries like the New England cod fishery are all but gone, it's hard to see how the aquatic civilization would have let this stand without warfare, as they are far more dependent on seafood than humans. Local fishers and seafarers would have also noticed them, especially if they were highly organized and with an advanced technology level. Whalers would have been very interested in them, not for actual hunting but because paying attention to big marine animals is their job. Look at Moby Dick, which was written by a guy who had whale-hunting experience, and the biologist Georg Wilhelm Steller, who worked as part of a whaling expedition. Both payed a lot of attention to large marine life.

This isn't even getting into sonar. Once international militaries start using sonar, or the marine civilization starts using sonar of their own to communicate (because if it works for whales and humans it would probably work well for them), they would be instantly found. At the very least by World War II, where the Allies and Axis would likely think the sounds of the aquatic civilization were signs of their respective enemies before finding their signals to be non-human. Indeed, something like this really happened during World War II with the invention of sonar. The big thing about sonar is it's really hard to miss because it's sound and hence is broadcast to everyone in listening distance, which is one reason why IRL whales are being driven nuts by being trapped in the realm of eternal dubstep caused by human sonar and why detecting enemy submarines by picking up on their own sonar is a thing.

There just really isn't enough productivity in the rest of the ocean to support a functioning civilization. The epipelagic zone is mostly sterile, the mesopelagic zone could at best support hunter-gatherers that did not use tools (so little different than whales), and the deep sea floor, despite having abundant mineral resources, just plain lacks enough food to sustain a civilization with no easy way to produce it (because you can't grow anything down there in sufficient quantities to feed a population).

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  • $\begingroup$ The only caveat is that there could be substantially more hydro-thermal vent colonies than we know about, so its possible there's a substantial about of deep sea biomass. There's almost certainly not enough to fuel a civilization though. $\endgroup$
    – abestrange
    Feb 16 at 16:22
  • $\begingroup$ Would a seamount help? Like, one in the middle of the ocean to make the distance to the sea floor small again, far away from any land? $\endgroup$ Feb 16 at 19:52
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    $\begingroup$ @CyrusDrake Probably not. The amount of life you could sustain on global seamounts would be about the same as modern humans are able to sustain on Pacific Polynesian islands, not really enough to sustain a global world-wide civilization. Zealandia might actually be a good option, it's a submerged region about the size of Saudi Arabia and over 90% of Zealandia has never been bottom trawled, but there's a big difference between a species hiding out on the Zealandia continental shelf and an entire civilization occupying... $\endgroup$ Feb 16 at 22:15
  • $\begingroup$ @CyrusDrake most of the Zealandian plateau. All it takes is one trawler trawling in the wrong location and your civilization gets discovered, because you can't move your buildings. It also wouldn't result in a world-spanning civilization with many nations like OP wants, at best you'd get the entire species concentrated in one area with a few far-flung colonies. $\endgroup$ Feb 16 at 22:18
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    $\begingroup$ @BlueTangsRock 1/3 of Zealandia is protected from fishing. Suspicious... $\endgroup$ Feb 18 at 17:31
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The ocean folks are very small.

The ocean is already big, and it is even bigger for these tiny entities. What detritus their civilization produces that could make its way to land or fishing nets is not recognized as products of intelligence. Fishing nets are easily avoided and submersible robots can be seen and heard from great distances and avoided. Their cities occupy only a few hundred square meters and only amazing luck (bad or good depending on viewpoint) would allow them to be discovered.

This solution fails if your fiction requires humanoid aqua-hotties in tight costumes. But actually only if you need human sized humanoid hotties. Tiny aqua people can be hot too!

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  • $\begingroup$ The species in the civilization would indeed shrink in size compared to the real world species they originate from, so this answer is a decent thing to consider. With my reasoning for the sizes of the different species in the civilization, species that originated from large ancestors such as dolphins, sharks, groupers, etc. would end up smaller due to their natural selection being in favor of less space and food requirement, likely making dwarfism in the "larger predatory" species quite prevalent throughout the civilization. $\endgroup$ Feb 17 at 19:31
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We, in our real world, are still deeply ignorant of what lurks in the depths of the oceans. We know better then surface of the Moon than the depth of the abyss above which we ship our containers.

The oceans are so vast that one needs to put effort in finding something, not in hiding it. Of many sea creatures we know they exist just because sometimes we stumble on their rotting corpses on our beaches, but we have yet to see one of them alive.

As long as they stay away from the intercontinental cables and oil rigs, they won't be noticed. Or they can always frame the sharks for chewing those pesky fiber optic cables running in the backyard of Qwstryqpi's home.

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    $\begingroup$ the real problem is feeding a civilization at that depth, the productivity of the ocean is miniscule at those depths. its like having a civilization that only lives in the deepest tundra. $\endgroup$
    – John
    Feb 16 at 19:03
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    $\begingroup$ The example of a giant squid washing up on the beach seems to run counter to the idea behind this answer - this is a perfect example of something that no one put any effort whosoever into finding, but whose existence was shown as clear as day simply due to natural ocean currents and dumb luck. I'd expect evidence of an undersea civilization to wash ashore at some point as well. $\endgroup$ Feb 16 at 19:33
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    $\begingroup$ @NuclearHoagie It's actually worse than that. A tool-making civilization would be having artifacts wash up on shore like crazy for the simple reason that one individual makes or uses many tools throughout their lifetime, similar to how shark teeth are disproportionately common in the fossil record because one shark can produce hundreds of teeth. Another good example would be giant deep sea tube worms (Siboglinidae). Despite living at the bottom of the sea, we've known they've existed since 1900. $\endgroup$ Feb 16 at 22:21
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They don't have to hide so long as they only live more than a mile below the water. Cameras or submersibles of any kind that can image below one mile are very rare, and curiously enough, subject to export control bureaucracy. If you see research being done down there, it's almost always with the Alvin. [apparently the Deepsea Challenger was destroyed by burning truck brakes? Odd...] So odds are no one would have found your civilization - even if they had no inkling they needed to conceal themselves.

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    $\begingroup$ the problem is that is the least productive parts of the ocean, feeding a civilization will be impossible there. $\endgroup$
    – John
    Feb 16 at 19:02
  • $\begingroup$ The reason such submersibles are subject to export control is the ocean bottom has immense strategic importance. The military keeps stuff down there, and think of all the incredibly important fiber optic telcom cables that you don't want messed with. $\endgroup$
    – stix
    Feb 16 at 20:12
  • $\begingroup$ If they had modern technology they would likely be using sonar or radio waves, and would get instantly discovered. Even if people weren't intentionally looking in the deep ocean, they'd notice something was coming from there. $\endgroup$ Feb 16 at 22:23
  • $\begingroup$ Most radio has problems underwater. If you were a marine mammal and used the sort of sonar the military uses at sea, your deafened and disoriented comrades would give you the sort of treatment ... they deserve. $\endgroup$ Feb 17 at 22:35
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I would recommend reading The Swarm by Frank Schätzing. A great sci-fi with a sentient species of single celled creatures that is governed by a hivemind that uses biotechnology and sea creatures to stop humans from destroying the oceans. https://en.wikipedia.org/wiki/The_Swarm_(Schätzing_novel)

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    $\begingroup$ Welcome to WorldBuilding.SE! Would you be able to explain in more detail how the linked book answers the question? Currently, this is reliant on the OP being able to track down a copy of the book. $\endgroup$
    – F1Krazy
    Feb 16 at 16:11
  • $\begingroup$ This would have been better posted as a comment under the question, rather than an answer. We expect answers to be self-contained, and explain why they are correct. What you've posted here is minimalist and without explanation. (From review) $\endgroup$ Feb 16 at 17:04

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