There is little if anything people devoted to science or logic can admit they can understand if they are told or see anything, in any way to learn, that brings there attention to what legends call the Sea-ones Island.
To start while all records of it description retains consistency of the base layout besides vegetation with the very centre of the Island having a mountain or Volcano shaped like a fish's tail sticking out from the ground complete with fin-like cliff-points long enough at the very top to complete the appearance...
...yet where Sea-ones' is upon the world shifts so frequently many claim it cannot really be a single Island. in the 1500s a sea-merchant and his crew all claimed to have chanced upon it 200 miles of the coast of China. 1632 a prisoner escort ship had guards professing a change of course for the captain's route was taking the ship into what was almost a crashing upon an island only to be reprimanded after an investigation the following year, their descriptions matching Sea-ones. 1860 a slave-trader's ship crashed and lost it's 'cargo' and half it's crew, those that were recovered alive mostly ended up in an insane asylum ranting the Island changed those lost... this however while not an open topic is closest to Sea-ones Island biggest secrets.
See if you were to somehow find yourself on Sea-ones Island you'd find it'll be more a wired mixture of sea-flats and tiny-inlets surrounding it's notable centre and that's if you don't meet, let alone join, the locals...genuine merfolk who's upper human bodies come from various ethnic origins...
...you see within the lower core of the fish-tailed mountain linked by a number of underwater and land tunnels is a bizarre pool like area that can turn humans into merfolk and has done so for countless generations, Maybe further back that the earliest recorded legends...and though somewhat rarer can turn them back or turn natural born (as in the descendents of past converts) into humans. yet none who come from the later if they were ever honest ever recorded how these transformations work.
That said it's no coincidence the Island is always moving, some returned from it claim it can move between worlds if that can be believed, for aside from some rare chance encounters they say the Island calls for people to join the merfolk; mostly those with powerful desires to abandon their human lives for the sea or believe in secret they have a calling for something beyond basic human understanding... whatever the case those that are 'chosen' to learn the full nature of Sea-ones Island are hardly ever seen again regardless of age.
Now somewhere within the timeframe of the 21st century or beyond who is now chasing upon or answering a call from Sea-ones Island: