Atlantis-type situation but in reverse. It goes up instead of down.
It is known that the sea is full of cities — as above, so below, etc. etc. We know this because we've gone to one of them- more accurately, it came to us.
Mer is a city designed for swimming thru, in much the same way that Heaven is designed for flying and New York is designed for teleporting. There are no streets, but there are parks. There is a distant sense of human beauty, beneath the abyssopelagic architecture. There are rows of whalebone columns, and gardens. There are bathhouses.
Sixty years ago, it surfaced just off the coast of Guam. It is carved almost entirely out of pumice. It is home to the mermaids.
In the city of Mer, humans outnumber mermaids 16-to-1. You might call this relationship symbiotic: mermaids have money (from deep sea treasures and contributions to the pharmaceutical industry) and humans have legs.
So it is that the most common mode of transportation in Mer is the man-powered palanquin. There have been pushes from the automobile industry to break into the mermaid market; none have held water.
And the livable quarters of the city have been retrofitted with spiraling stairways for two-legged folk and elevator shafts for the rest. The unlivable quarters are still under reconstruction, or have been preserved by the protests of the elders.
The elder mermaids remain, swollen blankets of pinkish flesh pouring out of inaccessible towers. (Their bodies were never meant for light, dry air, and low pressure environments.) They remember the day of judgement, when their homes turned to porous stone and the Seven Plagues of Air were set upon them. They watch the seagull-infested horizon (ech) thru milky, basketball-sized eyes, and rumble disapprovingly.
The greatest of the aquatic generation, Matriline Susubyr, pours from her laboratory, driven mad by the ascent. A powerful biomancer in her own right, she is the reason the Mer can breathe above water. She is also the reason for their banishment.
People have mixed feelings about her.
The younger generations forget that this life is their curse. They forget what they lost — the freedom of movement they enjoyed in the deep sea before their banishment, the weight of the sin that earned them its enmity. They just want to do TikTok dances and yeet a naynay.
So you want to play a Beached Mermaid:
Perk(s): You are 15'-45' long mermaid. You can take twice as much damage as normal and are immune to old age.
Quirk(s): You can't swim or breathe underwater. You must be hauled around in an appropriately-sized wagon.