MerpolisRuled by Alphea, Merpolis is the underwater city within the inland sea called the Rear Gulf. It is carved from the mountains of white stone and richly decorated by the artistic mermaids. It probably has more art and decorations than the rest of the world put together: it's literally filled with sculptures made of white stone, which is the main material for sculpting under the sea. Every home is a beautifully decorated cave, carved into the underwater mountains of white stone.
Underwater tourism is something that's available in Merpolis thanks to the special breathing gum, making it possible to visit the city for creatures that walk on land. The city itself is built in very shallow waters, so that whoever is out of their breathing gum can easily swim up to the surface to refill it. There's a lot of areas that are so very close to the surface that they can be used as pools for both the two-legged and the mermaids to play in together.
But Merpolis does also have a deeper level, where the Temple of Pisces can be found. This level is too deep for the two-legged to reach, but it has its very own miraculous creatures who also don't feel comfortable in the shallow waters. Because of this, the mermaids try to depict the deep ones in their art to let the two-legged know what these creatures look like, how they live and what kind of hobbies they have. The bodies of the deep ones are very soft, able to conform to the great underwater pressure, and their limbs often reach the state of tentacles in their softness and shapes. They have jelly-like skin and softer, sluggish features.
Thanks to A.F. for helping with ideas ^^