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?