Inspired by this answer to this question.

In my setting, there exist magical immortal mutants descended from humans who randomly became "magically active" and transformed into the first member of their own species, growing in number first by mating with humans and then with their own kind once said "own kind" reached critical mass. Several of these races are sea-dwelling, and in the process of working out their history, an interesting question came up. Given that a lot of sea life ideally wants to exist in water shallow enough for plant life to exist on the ocean floor, it makes sense to me that over time, the massively-underwater continent of Zealandia would become highly populated by merfolk and other such creatures.

Here's the problem: in the 1800s, when photography started really picking up speed, immortals resolved that they had to let human knowledge of them fade into myth and legend. This was all well and good for the immortal species who could pass for human and could blend into human society. But with the merfolk and other aquatic immortals, owing to the fact that, crucially, humans do not live under the sea, this mandate eventually made it basically impossible for the seafolk to live anywhere that humans would regularly be around, because obviously if humans ever found conclusive proof that anything sentient lived under the ocean, that'd be a huge secrecy breach. Wherever they lived, they'd have to be able to use the assorted magical abilities at their combined disposal to keep humans away from it.

So at some point in the past, the Zealandian seafolk destroyed their own underwater civilization and fled for other parts of the world. Some lived around remote seamounts. Some could pretend to be humans as long as they lived near water. Others fled into the unexplored and hostile abysses of the deepest ocean floors. But their ancestral home in Zealandia was no longer inhabitable due to human technology getting too powerful for them to be able to exist without being detected so close to human civilization.

What would be the breaking point for this civilization? What would be the earliest technological advancement that would make it impossible, or at least extremely dangerous, for aquatic sentient creatures to live in the waters of Zealandia without themselves or their crude structures being discovered by humans?


2 Answers 2


Technologically, it would probably be sonar. Sonar is when humanity really started to look around underwater, rather than just dredge the bottom opportunistically or using sounding ropes to measure depth. Sonar really took off during World War Ii, and the ocean-going technology developed during World War Ii eventually led to things like the Alvin and the discovery of the mid-ocean ridges. Indeed there is a real-life parallel. When people started using sonar they found that the ocean had a "false bottom" that moved based on time of day. It turns our to be sonar scattering from massive densities of mesopelagic fish that no one knew existed.

SCUBA, invented about the same time, would also be a huge threat. The first modern SCUBA system, the Aqua-Lung, was developed in 1942-1943. The 1930s was really the period where we stopped looking at the surface of the ocean and started really looking at what was underneath. A lot of the earlier bathyspheres, submarines, and breathing apparatuses were crude and easily avoidable.

More broadly though? The Kanaks and the Maori would have noticed they were there, from tools washing onshore if nothing else, I don't know how much either fished in the open seas. The broader world wouldn't have known this given New Zealand and New Caledonia are relatively isolated and don't have extensive contacts with the rest of the world, but once Europeans made contact they would find out pretty shortly. James Cook's expeditions to New Zealand in 1769 and New Caledonia in 1775 would have been the writing on the wall.


Your main problem is that 'Zealandia' disappeared long before humans evolved on Earth

Geological records indicate that the 'lost' continent of Zealandia submerged entirely about 23 plus million years ago. Long before there were any primates let alone humans on Earth.

The islands of New Zealand then slowly began to emerge shortly after 20 million years ago. But they were still small, isolated, volcanic and vastly separated in distance from where humanity evolved in Africa where no true humans appeared about 200 thousand years ago! And humans didn't reach the vicinity of Zelandia (Australia) until about 40-50 thousand years ago and New Zeland itself 600 years or so ago! By which time 'Zealandia' was about 3000 to 1500 meters underwater! That is not 'shallow'.

And even allowing for the fact that mer-people somehow managed to swim there and found a civilization in those shallow??? waters there would simply be nothing for them to build a 'civilization' with except sediment/mud and the occasion loose rock! I mean what are they going to do? Rub two sticks together to make fire?

IMO you somehow have to fast forward the evolution of these creatures to a time period and location where humans were present and 'civilized'.

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    $\begingroup$ Why would it need to have been above the surface more recently? The entire point of them moving there is for the abundance of shallow water. $\endgroup$ Feb 17, 2021 at 13:14
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    $\begingroup$ That's the point of them living on Zealandia. They're merfolk. The fact that Zealandia is a large, shallow marine platform with abundant resources is ideal for them. $\endgroup$ Feb 17, 2021 at 15:50
  • $\begingroup$ As I noted Zealandia is 3000 to 1500 meters below the surface of the ocean on average (barring the immediate coast of New Zealand itself. That's way to deep for them to get any 'significant 'resources' they couldn't get anywhere else in the world at at the same depth. Basically if they are living at those depths they can live anywhere in the world they want. And its not shallow. The Dogger Bank is shallow, the Baltic is shallow, The Black Sea is Shallow etc etc and all of them have traces of previous human settlement. Zealandia doesn't. There are lots of better locations to chose. $\endgroup$
    – Mon
    Feb 17, 2021 at 20:36
  • $\begingroup$ @Mon OP want a location where they can avoid human detection as long as possible. Dogger Bank, Baltic Sea, and Black Sea would have them well-known to the world by the early Bronze age given the high settlement around there. Zealandia is as big as all three listed areas combined. Zealandia also has significant mineral and gas resources and substantial fisheries with high marine biodiversity $\endgroup$ Feb 18, 2021 at 5:53
  • $\begingroup$ The locations I mentioned are where their civilizations could have developed via interaction with human beings. There are other locations of the coast of China and elsewhere that would serve as well. The point is firstly they won't have civilization until humans do - at least not without interacting with them. Secondly, apart from fish all the other resources you've listed require advanced technology to exploit. Minerals and natural gas etc cant be exploited without access to metallurgy, chemistry, forges & other technology you can't produce underwater, you cant forge a steel tool in water. $\endgroup$
    – Mon
    Feb 18, 2021 at 10:14

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