I'm trying to remember the name of the fantastic book about an underwater world.
Date: unknown, I read it in the summer of 2013, but the book looked pretty old
Races: people and humanoid dolphins
The history narrates about the people living at an ocean floor in certain capsules which remained from the previous generations and which technologies are lost. People live in the small village because the quantity of capsules is limited. Near the settlement there is a crack in the ocean floor - very wide and deep. People in the settlement call it the Abyss. Nobody knows what is behind the Abyss or at its bottom, because nobody can cross it due to strong flows which carry away people into impenetrable depth. Therefore people in the village teach children to never swim far away.
In the settlement there is a strange ceremony - when a child is born, the newborn is dumped in the Abyss. The majority of children then come back in an air bubble, but some don't. After a ceremony, children gain ability to remain longer under water. It is considered that the Abyss chooses whom to let live and to whom to make die.
The main character of the book is a young girl. Some reasons which I don't remember have forced her to try to cross the Abyss, but she didn't manage to, and the flow has carried her away into the depths. She has regained consciousness in a cave where there was breathable air. There she has met humanoid dolphins about whom nobody knew before. They could swim long under water, and also to ineptly walk on a surface and to breathe air. Dolphins have taken away the girl to the city which was much more advanced in the technological and cultural plan. As it becomes clear later, dolphins rescued the children thrown into an abyss and whenever possible sent them back, but sometimes left them to live in the city.
City walls were decorated with engravings and mosaics from which the girl learns about world history, about ancestors and what advanced science they had.